Sarah Jane Doyle's mother was one of the few who didn't take to him. She wasn't sure why - something to do with the presumptuous way he addressed her and her husband by their first names, maybe. But to everyone else - gardai, lawyers, the psychiatrist, Sarah Jane's father, even - he was a grand fellow, variously described as "very agreeable . . . very plausible . . . pleasant . . . mannerly . . . quiet . . . intelligent . . . unargumentative . . . willing to ask for and take advice".

The perfect gentleman.

It is obvious why he seemed just the "tonic" Sarah Jane needed when she spotted him in a Dublin nightclub in April 1997. With all the pathetic vulnerability of an early school-leaver, just gone 18, with a three-week-old baby by a man who "wasn't around", and suffering from a combination of stretchmarks, shame and depression, she thought no one would want to know her. And so, as she told RTE this week, she latched on to this good-looking bloke, the one prepared to show her affection.

When they got chatting, she discovered that he too, had a baby, but that his relationship with the mother, Lucy Porter, was over. She also deduced that his mother was Irish and that he had been raised in Huddersfield. That part was true.

He was born in Ballina, Co Mayo in 1973, though his mother returned to Huddersfield a year later and settled in the suburb of Bradley, a vast collection of local authority estates and - nowadays - an increasingly healthy mix of private and public developments.

After that, however, the details grow murky. His father has been variously described as Jamaican and as "an Italian businessman". To begin with, Nash tried to create the impression that he had wealthy parents.

Later, Sarah Jane told the Star, he drunkenly blurted out that his father had left before he was born and that his mother used to take it out on him, beating him with a vacuum cleaner or a stiletto shoe. Later still, he would tell psychiatrist Dr Richard Blennerhassett that deep down he believed he was a "trick baby", the unwanted by-product of a prostitute and her client.

His mother never loved him as a child, he claimed, nor did he love her. By Nash's account, his maternal grandparents, with whom he had spent some time in Ballina, disappeared from the scene when he was about seven. Around that time, he claims, he began to smoke cannabis.

After primary, he moved on to All Saints, the Catholic high school in Huddersfield and it was then, he claims, that he sought help from a relative and was turned away.

Though considered bright, he left school at 16, picked up a conviction for minor assault at 17 ("pushing a girl" is how he recalls it), and left home at 18 after a series of disagreements with his mother.

From there, he moved into a succession of flats and bedsits and further into the twilight world of petty criminals and drug dealers. In 1995, he got out of Huddersfield after falling foul of a local criminal and went to Leeds where he moved in with Lucy Porter and got a job in telesales (a job he would also do in Dublin).

Once again, he came to police attention when he was charged with possession of cannabis with intent to supply and was released on bail. Within weeks, the couple had moved to Dublin where their daughter was born in November 1996. But not long after, according to Lucy, he was already showing a vicious streak.

On one occasion, he caught her by the head and banged it off the corner of a sofa. He tried to strangle her and threatened her with a steam iron. He threatened to kill her when he found out that she had told the friend who had lent him a video recorder that he had sold it. Later he would deny "ever laying a finger on her" though he recalled "her banging her head against a sofa. . . I think I was holding her at the time", he added.

This in any event, signalled the end of that relationship, times he would later sum up as "very hectic". Just a few months later, on April 1st, 1997, he was floating around a Dublin nightclub alone, the object of a fragile teenager's fancy.

Within weeks, Sarah Jane Doyle had moved in with him in Prussia Street, where Lucy, awkwardly, was still in situ. The couple ended up on Clonliffe Road where they shared with two housemates.

"Things were fine. Mark and I got on well at first," she said, although witnesses would tell gardai of violent rows between them - with one witness in particular asserting she had seen bruises on Sarah Jane's face. Nonetheless, the row that would give her a real taste of Nash's rages - and the scene that would later give the prosecution a motive of sorts - took place on August 13th, just two days before the fateful trip to Roscommon. She wanted to move closer to the family home, after her brother Richard had died of a drugs overdose in June. After fighting about it on the phone, he arrived home still shouting.

When one of the housemates intervened, he "screamed" abuse at her, upsetting her to the extent that she left that night. Upstairs, he kicked in a television set and punched the wall. It was after that that Sarah Jane said she wanted to go down the country for a break and visit her sister, Catherine. (In fact, she told a friend, she hoped Catherine could help him). He promptly agreed.

Is it possible that - as the prosecution suggested - he stored up this rage for two days? That he took his baby daughter (along with Sarah and her baby son) to a rural backwater where he could bludgeon Sarah - already pregnant with his child - with an iron lever, leaving her for dead, blood-soaked, and barely able to crawl through the grass to get help from a neighbour? That this could be why he set upon Carl and Catherine, dopey on cannabis and alcohol, bludgeon and strangle Catherine, and using a different boning knife for each, leave 16 deep, gaping knife wounds in Catherine's body, and five in Carl's, the latter inflicted with such ferocity as to leave fragments of the broken knife stuck in his shirt?

If this seems far-fetched as a motive, then the question remains: why did he do it? There was evidence that he spent a highly uncomfortable 45 minutes in the toilet with vomiting and diarrhoea and that it was during this time that he felt a desire to murder descending on him like a "cloud".

Is it possible that he was more upset than he seemed when Sarah and Catherine took photographs of him in that humiliating posture in the toilet? He testified that he "just lost control" and doesn't know why. Sarah Jane, the only other adult witness left alive, agreed. She had "no idea", she said, why he turned from a Dr Jekyll into a Mr Hyde.

But turn he did. As he climbed the stairs just before the attack, he had "mad eyes" she said and a "weird" look. As he was bludgeoning her, she asked him why: "He told me I had to die".

He agreed with his counsel, Greg Murphy, that there was no reason whatever for the killings: "That is the way it happened. It was a mixture of drink, being drunk, being sick, seeing a knife and paranoia".

He made no attempt to dispose of evidence; one knife was dropped in the house, another in the garden. It is believed that he wanted to plead guilty at the outset, to save Sarah Jane and her family from the ordeal of a court case.

So by any standard, it seemed, the man was bonkers, as his senior counsel put it in a powerful closing speech. "To the normal man in the street, he is bonkers, daft as a brush." Interestingly, this is almost precisely the definition of an abnormality of mind as adjudged by the British Lord Chief Justice in 1960, "a state of mind", he said, "so different from that of ordinary human beings that the reasonable man would term it abnormal".

Adding fuel to this perception is the fact that Nash, while on the run in August 1997, became the second person to confess to the savage, motiveless murders of two elderly psychiatric patients in Grangegorman the previous March, shortly before he met Sarah Jane.

This confession was sufficiently detailed to provoke such a bout of Garda heart-searching that the first "confessor", Dean Lyons, was released after nine months in custody. Nash, however, subsequently withdrew his confession and so far, there has been no move to charge him with those murders.

There was also the case of Dorothy Wood, an elderly woman murdered in Huddersfield in the spring of 1996. On the word of a local petty criminal already charged with the crime, British investigators travelled here in September to take Nash's fingerprints. Huddersfield police said this week, however, that they have eliminated him from their inquiries.

So is Mark Nash "bonkers"? No, said the only expert witness allowed to speculate in court. Dr Richard Blennerhassett, clinical director of psychiatry at St Ita's, Portrane and at Beaumont Hospital, concluded that he suffered from no psychiatric disorder "as we would define it" at the time of the crime, although he agreed that Nash might have had a "subliminal rage" and not have known what he was doing.

"It's possible that he was not entirely in a rational state of mind so that he was not fully aware of his crime". Having interviewed Nash for two hours before the trial, he was at pains to point out to The Irish Times that he has "a high degree of scepticism about all of Nash's statements".

His impression of the accused man was of a "very intelligent, articulate individual who can be very, very persuasive . . . a very controlled, very confident individual". Alongside that, however, "is the degree to which consciously or not, Nash has built up an image of himself and projected himself as a cool, street-smart, John Travolta-type character (as in Pulp Fiction), the type where nothing gets to him . . . And underlying this is the way in which he considers himself perhaps really quite a worthless individual.

`If there is truth in what he says about his birth and early relationship with his mother, then that scenario would not have contributed to a healthy sense of development. Behind that exterior is this angry, deprived child whom no one cared for and who has rage against the world and which he may not even be conscious of . . .

"In the same way, respect is critical to Mark Nash, in the whole way he views himself and the way he expects others to behave towards him. He has a very keen sensitivity to slights directed towards him.

"So we have this guy in this house on this tragic night, drinking, taking cannabis - which have cumulative effects, and affect the way we think and feel - and we have his character make-up linked to the disinhibiting effect of the drink and cannabis . . ."

As for motive, Dr Blennerhassett notes that we are missing "a fuller account" of what happened that night. There is much that Nash apparently cannot remember.

And though the doctor, naturally, refuses to speculate, the question hangs in the air: did Mark Nash - to whom respect is critical - get the respect he felt he deserved that night? Remember the photographs on the toilet.

In any event, he still passes the acid test: he still seems utterly "bonkers" to the man in the street. So supposing the jury had accepted his counsel's contention on Monday and concluded that he was indeed bonkers?

In law, they would have had no choice but to acquit him; in human terms, an impossible choice for many jury members, given the real probability that he would reoffend.

The problem, as Greg Murphy pointed out in his speech, is that no legal halfway house between murder and manslaughter - that of "diminished responsibility" or "irresistible impulse" - is available in Irish law. This in spite of the fact that the Government has been considering it for 40 years.

The bottom line here is whether Mark Nash - and hundreds like him - are suitable candidates for ordinary prison life and eventually, for release without treatment. Or will it finally take an aggrieved victim or bereaved relative suing the State to persuade it to stop "considering" and reach a decision?

Above all, for society's sake, the case of Mark Nash should reopen the whole debate as to how a human being will develop and how much of this is predictable.

"Generally, disturbed behaviour is easily picked up in the early stages, at nursery school and places like that", says Dr Blennerhassett, who is back in Ireland a year, "but we - in the UK and Ireland - are not organised in finding ways to help problem children. There are very few places you can turn . . . In Ireland we have very poorly developed psychiatry services."

The child of Mark Nash and Sarah Jane Doyle is now nearly six months old, "a gorgeous, petite, blue eyed, fair-haired child with skin like porcelain", according to the besotted mother. But a child, nonetheless, who may never know her father.