IN HIS house on Howth summit, Conor Cruise O'Brien has a yellowed photograph of his great uncle, Father Eugene Sheehy, a turbulent priest notorious a century ago for his Fenian and Land League sympathies. On it is an inscription "Eisean a mhuin an tir ghra dhom" ("He taught me patriotism").

Father Sheehy was the parish priest of Bruree, Co Limerick, where one of his most devoted parishioners was the young Eamon de Valera. When he was president, de Valera presented Conor Cruise O'Brien, then a distinguished diplomat for the State, with the photograph of hiss ancestor. At the time, it seemed to bear witness to the continuity of Irish nationalism over the best part of a century.

Listening this week to Robert McCartney describe as "a great coup" Conor Cruise O'Brien's decision to stand as a candidate for his United Kingdom Unionist Party in next month's Northern Ireland elections, it was hard to imagine what de Valera might write on the photograph now.

The last time there were negotiations with a reasonable chance of success, at Sunningdale in 1974, O'Brien was a member of the Irish government team. Next month, even if he does not win a seat in the forum, he will, he says, serve as an adviser to McCartney in the negotiations, "when constitutional questions and stuff like that is under consideration". The continuity attested to by the photograph has been broken in the most dramatic manner imaginable.

At one level, however, his decision to stand as a unionist seems an appropriate enough conclusion to O'Brien's public career. He himself talks of it as something that was, in retrospect, "bound to happen".

That it should seem inevitable that a former Irish government minister will end up as part of a team negotiating, in a sense, against the Irish government says much for the direction of that career.

There is certainly a logical progression from his fierce opposition to the IRA in the early 1970s to his critique of the intellectual traditions of Irish nationalism over the last 25 years, to his willingness to address the Friends of the Union at Westminster in 1990 and his support for Robert McCartney in last year's North Down by election.

"It's certainly not for the sake of wanting to go back into politics", he says of his candidacy now. "I've been very happy in my retirement from active politics, and just commenting from time to time, but I have been getting increasingly anxious as to where we're all going in the nationalist co operation to squeeze the unionists in a direction in which they refuse to go.

"I have criticised the national agenda, also known in Gerry Adams's title as the Irish peace process, in print. So when Bob McCartney invited me to join his party, which will be opposing this same agenda capably and efficiently and moderately, I didn't feel I could refuse that invitation."

His admiration for his new party leader is such that he professes himself "entirely at home" in his political company. He admires, in particular, McCartney's efforts to distinguish unionism from Protestantism.

"When I went out on Bob's campaign last summer", he recalls, "he made, I think, two speeches on a strongly anti sectarian line. Now this was interesting because he was running against the Official Unionists, but there was no DUP candidate. And Bob's best chance was to get DUP votes.

"Now the best way to get DUP votes would not normally be thought of as making anti sectarian speeches, but he did. I thought that rather courageous."

AS A candidate himself, he will support McCartney's attacks on sectarian unionism but, he says, laughing, "I imagine he himself wouldn't want me to support him too loudly. I'll give just as much support as he feels he needs."

He acknowledges nevertheless that, in spite of his long standing criticism of the nationalist cause, there will be some lingering unionist suspicion of him. He has, he says, "a Paisley certificate of Irish nationalism" and "it will certainly turn off some people.

"But on the walkabouts last summer in North Down I was pretty widely recognised, and those who did recognise me came up and shook my hand and said they were glad to see me."

For someone who has always been willing to put his beliefs into action, his candidacy now is therefore entirely consistent. But it also involves certain paradoxes.

He criticises what he calls the Republic's "obsessive commitment to meddle and be seen to meddle in the affairs" of Northern Ireland. Yet his own candidacy, even though it is at the invitation of unionists, is certainly an intervention in Northern politics from south of the Border.

Equally, his disavowal of Irish nationalism is based on his belief that it is ultimately inseparable from Catholicism, yet he standing for a party which believes that unionism can be separated from Protestantism.

He denies, however, that there is a contradiction between one position and the other. "In theory", he argues, "both should be easy, because the old religions don't have the grip they had on both sides of the Border. But I would still be inclined to isolated a tribal sectarian component and as the sectarian one gets weaker the tribal one may get stronger.

"Certainly the antipathies and animosities between the two sets off people don't seem to be diminishing. I think they might diminish if we would leave Northern Ireland alone for a bit."

At a deeper level, his candidacy certainly represents a fundamental change in his perception of his role as an intellectual in politics.

In his valedictory lecture at New York University in 1969, before returning to Ireland to enter active politics, he explained the role he would go on to play in the Republic "The intellectual should not voluntarily forsake or despise his clan, tribe, or nation. He should be, if he can, the antennae of his tribe, among their means of understanding their relation to other tribes and to the world."

In his critique of Irish nationalist assumptions, he certainly played the role he had identified for himself. He did act as the antennae of his own tribe, picking up and sending on often unpalatable messages about the validity of cherished assumptions.

Deeply affected, both personally and intellectually, by his experiences as United Nations representative in the breakaway Congolese province of Katanga in 1961, he brought a deep horror of violence and instability to his analysis of the emerging conflict in Northern Ireland. While remaining within his own tribe, he subjected many of its most cherished beliefs to a powerfully destructive analysis.

NOW that he has broken the rule he laid down ink 1969 and voluntarily forsaken his own tribe, will he try to act as the antennae of unionism, passing on messages that his adopted tribe might not want to hear? Will he do what he did with Irish nationalism, and subject the assumptions of unionism to the same astringent interrogation?

"I don't really feel," he says, "that I have to give that priority. I feel that the unionists are essentially besieged. They're under a pan nationalist siege, of which the main stimulus is being supplied by Sinn Fein.

"So I don't see it as my job get out and annoy the unionists. I think they're being annoyed quite enough. I have, in an Ormange context at Orange House, urged them to see that marching through areas that don't want them is not good policy for the Orange Order itself or for the unionist community. And I got quite a good hearing for that. But in speaking for and to the United Kingdom Unionists I don't have to labour that because they have no connections with the Orange Order."

Likewise, he does not accept that the really important challenge at the moment is not to move from nationalism to unionism, but to move beyond both. "The main pressure," he says, "is coming from nationalists. It's dangerous, not just for the North but also for us. In that context, I'm more useful taking a stand with unionists than I would be on some kind of neutral ground."

He has just one small regret about his decision. "I have no qualms or heart searching, though I would have had six years ago certainly. But I am a bit sorry for decent people who have been defending me for years among their friends and saying that I'm not a unionist, as I hadn't formally said I was.

"I'm sorry to let them down, in a way, and I hope they don't feel too badly. Otherwise, for myself, I'm quite happy about it."