In his book States of Ireland, Dr Conor Cruise O'Brien outlined two political scenarios for the future of the North: one benign, the other malign. Likewise, there are two political views of "The Cruiser" himself. A man who worked with him in the United Nations expressed the malign one to me: "There was always trouble wherever that man went." The benign view is that he is a courageous and forward-looking thinker who blows the whistle on dangerous trends in society no matter what the cost to himself.

If the man takes pleasure in stirring things up, he certainly wasn't showing it at his bungalow in Howth yesterday afternoon. Though reluctant to discuss his resignation from the UK Unionist Party in detail, he was clearly disappointed that circumstances had forced a breach.

It was probably Mr Michael McGimpsey who forced him to quit in the end. At Monday's session in Stormont, the Ulster Unionist Assembly Member sat behind the UKUP leader, Mr Robert McCartney, holding up a copy of a Sunday newspaper article in which O'Brien outlined a scenario where unionists might have to negotiate for a united Ireland with constitutional nationalists rather than fall under the sway of the Provisional IRA.

It's a subtle argument, but there isn't much room for subtlety in Northern politics right now with all-out political strife between the different factions of unionism. Although McCartney took a tolerant and even good-humoured view of his colleague's hypothesis, O'Brien says other senior party members were "very irate indeed" and put pressure on the leader which coincided with the pressure from the party's enemies among the Ulster Unionists.

So was he resigning to prevent embarrassment to the party? "That's it, or rather to check the development of embarrassment. The embarrassment has already been caused, as we saw in the proceedings of the Assembly, where Bob's enemies have latched on to what I have said in order to damage him. In all his dealings with me, he has shown great forbearance and kindness. He didn't ask me to resign or want me to resign but he saw eventually that I would have to. I proposed it and he finally agreed to it."

Seasoned journalist though he is, O'Brien admits he miscalculated the likely media treatment of his views. "Yes, I didn't realise quite how much it would be blown up. The blowing-up always involves some slanting or tilting or whatnot and it was made to appear as if what I had made was an unconditional prophecy. I was merely warning that certain things were likely to happen."

So was this a sad and emotionally upsetting day for him? "No, it's not, because I have gone through it knowing that we remain, and always will remain, close friends with the McCartneys. One of the things that had worried me most was that this might disrupt our relationship but there's no question of that now."

One of the more striking episodes in O'Brien's career was his decision in the early 1970s to challenge the assumptions of Irish nationalists and their insufficient empathy with unionists. He got little thanks and it probably cost him his Dail seat in 1977. Now he is presenting the unionists with equally unpalatable views and opinions and there is a similar lack of gratitude.

"It is the duty of an intellectual in politics to examine the changing realities of the situation, because realities do change." He feels he should have resigned from the UKUP before his current analysis, taken from a forthcoming book called Memoir: My Life and Themes (Poolbeg £20), was published.

His forecast of events may be unappealing to unionists, but not his views on the Provisional IRA, which he regards as a fascist organisation working its way towards supreme power just as the Nazis did in the Germany of the 1930s. With feeling in his voice, he says: "They are haters. Most of us, with all our faults, are not great haters. These people are seething with hate, it drips from them. You can see it even in their more pacific statements. They are now feeling their own strength. They have become politically cunning, much more politically effective than other parties on the island, all of which spells no good at all for all the rest of us. The time will come when a halt will have to be called."

Nevertheless, he now sees the declared aim of the Provisional IRA - a united Ireland - as something the Protestants of Ulster may have to consider as their only means of survival. There are two likely developments he believes will bring this about, the first being the seating of Sinn Fein ministers at the cabinet table in the North without any disarmament by the IRA. "And the second and worse being the `reform' of the RUC at the demand of the people who have been murdering members of that force. It was only if those two forebodings came true, which I fear they will - and in particular the second - that I thought unionists should seriously consider the option which I have put forward. I still think all that and I will continue at appropriate moments to say so and draw inferences as things go on."

I put it to him he had come full circle from the time he used to write anti-partition propaganda as a young civil servant in Iveagh House. "That's not really the case and when I was writing pamphlets about a united Ireland I was doing so as a functionary of the Department of External Affairs, as it was then, and doing the bidding of Sean MacBride [minister at the time]." He is "not terribly proud of that chapter in my life" and will be saying so in his forthcoming book.

O'Brien was one of three members of the UKUP elected to the Northern Ireland Forum in 1996 but his participation in that body was interrupted when he had a stroke. He did not stand in the Assembly elections, partly for health reasons. "I think the stroke I had was perhaps to some extent precipitated by the strain of the unpleasantly contentious proceedings at the Forum, which were very fraught indeed, and I certainly didn't want to be involved in that again."

His controversial and, some would say, farfetched hypothesis holds out the possibility of eventual fusion between the RUC and the Garda Siochana. But wouldn't this involve, among other and more serious considerations, losing the "royal" from its name? "I don't think they would object to dropping the `royal' from the title. The whole thing they do object to and would be demoralised by is consenting to changes at the demand of the people who have murdered hundreds of their members over 30 years. This is a really horrible thing to contemplate doing. When it gets to that point it is necessary to examine a lot of options that people don't want to look at now."

I asked him if the end-product would be a "32-county Ulster" or a united Ireland similar to the one that existed under the Crown. "It would be an independent Ireland in friendly relations with Britain and without any ties with paramilitaries, in fact setting its face more firmly against both sets of paramilitaries than is now the case. To me, the main attraction is it would get rid of what is very nearly Sinn Fein-IRA dominance of the political process in this island, which is a horrible thing."

He was becoming tired, which he joked was a dangerous condition to be in when giving interviews. I left him, a copy of his resignation letter in my hand. He may be 81 next Tuesday but the Sage of Howth is still controversial after all these years.