With sprightly prose by lumbering logic, Conor Cruise O'Brien the editor of the Observer of London, leveled another broadside against American politicians who are pressuring to create a united Ireland.

Writing in the New York Times a few days ago, O'Brien charged that Americans of Irish Catholic descent like Sen. Daniel Patrick Moynihan and Gov. Hugh Carey are "chronically blind" to the realities in Northern Ireland. The sightless, it seems, don't face "one neglected fact: The great majority of the population of Northern Ireland want to remain in the United Kingdom."

This argument, which has the aura of democracy that blends well with the tone of high-minded reason that the British government imagines itself to imbue, is hollow. Democracy isn't only about majority rule. Minority rights are crucial, too.

This is what most of the American politicians have been stressing. Human rights' violations against the Catholic minority have been so severe and so persistent that looking away and keeping up the pretense that British propriety would never allow such abuses is almost a form of complicity. Americans outraged against human rights' violations in Argentina or Korea are as right to speak out against those governments as against Britain's.

It is meaningless to point to "the great majority" in Northern Ireland that wants to maintain ties to Britain. Of course it does. Discrimination and brutality against the Catholic minority assures the economic and social well-being of large numbers of Protestants. Jobs, for one thing, are at stake. Asking the Ulster loyalists if they want to change their political structure would have been like asking the whites of Mississippi in the early 1960s if they wanted to change their methods of brutalizing the blacks.

The effect of the British government's continued presence in Northern Ireland is to provide the Northern Ireland loyalists with the security of domination over a minority. To begin the process that would lead to eventual withdrawal, the Thatcher government would be forcing the Ulster Protestants to confront their diseased system.

At the moment, attention to this disease has been diverted by the resurgence of Irish Republican Army terrorism. A condemnation of the Mountbatten killings was the only proper response from anyone concerned about peace in Northern Ireland. But in the mind of Conor Cruise O'Brien, the pressure against the British government from concerned Americans has the effect of giving encouragement to "the IRA to keep up the killings." A "sort of political-military firestorm" is created, he imagines.

Here again, OBrien relies on the shopworn. It is too pat, and too convenient, to brand American observers as unwitting intellectual allies of the IRA. Word has long been out about the thugs and young toughs that comprise much of the IRA. They don't have wide support in Ireland, north or south, and they have even less here.

To keep on condemning the IRA and let it go at that is to help along the illusion that the terrorists are the sole cause of Northern Ireland's chaos. They aren't. As Jack Lynch, the Irish prime minister says, "Unless we can produce a situation in which the community in the North can give allegiance to a formal administration, then this kind of violence will continue. It is a question of getting at the cause of the matter, not the effect."

The latter notion about cause and effect has long been the thinking of the Irish National Caucus, the Washington-based lobbying group that has been influential in getting Congress to see Northern Ireland as a human rights' issue. But the more difficult matter is whether or not the sentiment for British withdrawal among many American politicians can grow. The specter of civil war in Ireland is raised if Britain goes. O'Brien believes this, as do others who somehow think that the Irish, north and south, aren't up to the political challenge of uniting their island.

With 2,000 deaths in 10 years and incalculable suffering to numberless families, it is hard to imagine that the Irish want any more of it. An announced plan of orderly and gradual withdrawal of British rule is more likely to calm emotions than to inflame them. At the least, it would be enough to create the conditions for a dialogue to begin. Exceptions have occured, but it isn't the usual case that civil discourse breed civil war.