Prospect of N. Ireland unity dims as Catholic party boycotts assembly

The prospect of a political settlement in Northern Ireland in the near future appears extremely limited after the decision of the majority Roman Catholic party to abstain from taking their seats in a new assembly.

The Social Democratic and Labour Party (SDLP) voted 25 to 14 to participate in a special Oct. 20 election for the new assembly, but not to take their seats in the proposed 78-seat body to be set up by the British government.

The vote might appear to be an Irish approach to a typically Irish problem, but it has its own cold logic. The SDLP argued that the assembly offered nothing substantial to the Catholic community. There was no ''Irish dimension'' that satisfied Catholic aspiration for a united Ireland.

There was also no firm commitment that they would be allowed to share power in the north with the province's Protestant politicians.

The British government privately hoped that power-sharing would evolve, but there were no signs that the Protestants' representatives would allow this to happen. Ironically, the SDLP achieved power-sharing, and an Irish dimension, in British proposals hammered out eight years ago. But these plans were abandoned in the face of massive Protestant opposition.

This time the Catholics were offered much less, so it is no surprise that they have refused to participate. Instead they have presented fresh proposals to the British that have little chance of being implemented.

Britain's Northern Ireland secretary, James Prior, was stoic about the latest political threat to the assembly, which is his brainchild. He conceded that the SDLP decision was a setback but emphasized that the election will continue.

''You cannot alter the rules as you go along. You put down the framework and stick to it,'' he said.

The Unionists, never a group to miss a political trick, were quick to ask the British to scrap the provisions for possible ''cross-community support'' (the new term for power-sharing) in the assembly, once the SDLP had announced publicly that they would not take part. The government resisted this but even the greatest optimist gives the assembly little chance of success.

The leader of the moderate Alliance Party, Oliver Napier, reflected on the SDLP decision more in sorrow than in anger: ''Just because progress will be slow and difficult, this is no reason for abandoning the fight.''

The ''realpolitik'' of the situation is that the province is now saddled with an election campaign that threatens to be bitter and divisive. On the Catholic side, the SDLP is anxious to retain its electoral appeal in the face of opposition from provisional Sinn Fein - the political wing of Irish republicanism - and other minority parties.

On the Protestant side, there will be a fierce battle between the Official Unionists and the Rev. Ian Paisley's even more hard-line Democratic Unionists. Each is anxious to claim that it is the authentic leader of Ulster Protestantism , Unionists of all shades want to maximize their vote in order to pressure the British into conceding majority rule. This would mean that the Catholics would have no equal share in running the affairs of Northern Ireland.

No British government is likely to allow the province to be ruled on the Protestants' terms alone, so the deadlock should continue. Unless there is an unforeseen change of attitudes, the new assembly will wither away in sectarian verbiage.