Answer to Irish terror

The British government has offered Northern Ireland new political conditions to reduce the likelihood of future tragic events like those perpetrated by the Irish Republican Army in London on July 20. Must there be further major violence before Ulster politicians accept the decisionmaking responsibility being placed before them?

The warning from James Prior, Britain's secretary of state for Northern Ireland, is clear:

''We must get to the stage where people are responsible for their own decisions or else the extremists will flourish.''

Mr. Prior spoke these words in announcing the date, Oct. 20, for elections to a new 78-seat Northern Ireland assembly. This would be the beginning of an opportunity for the Northern Irish to gain more and more control over their own affairs.

Proposals for how executive powers should be exercised would require at least 70 percent agreement in the assembly in order to be submitted to Parliament. This would indicate acceptability to both the Protestant majority and growing Roman Catholic minority in the Northern Irish political community. Eventually a large range of legislative and executive powers could be devolved without giving up any more direct British rule than desired by the Northern Irish themselves.

Starting and sustaining such a system would not be easy. Compromise would be needed on all sides. The way one side or another can undermine power-sharing home rule efforts was indicated a decade ago.

But local politicians will have only themselves to blame if this initiative fails, too. Instead of resisting it or calling it unworkable, as many are doing, they ought to plunge in and show that they are capable of making something work.

The promise of Secretary Prior's plan lies in its whole approach of letting Northern Ireland fashion the decisions it wants to live with. It can go slow or fast. There are provisions for ''rolling back'' devolved powers to the secretary of state if broad support does not continue.

Mr. Prior might be excused for challenging the provincial politicians as President Reagan challenged the US Congress - if you don't like my plan, show me something better. Indeed, Mr. Prior's impending visit to the United States gives Washington an opportunity to lend moral support to his inventiveness and persistence in seeking political solutions in Northern Ireland.

Both Congress and the White House might also take the occasion to join in condemnation of IRA violence without any of the equivocations or hints of toleration that some individuals have been guilty of. For, while political solutions are necessary for long-range peace, there is an immediate need for checking terrorism.

Violence has not all come from one side over the years. But, with the outlawed IRA resurgent after a Falklands crisis pause, the focus is squarely on deterring it through international efforts.

The Irish Republic's role is essential here. It has been working with Britain more than in the past. This month, for the first time, an Irishman was convicted in a Dublin court for an offense (possession of explosives) committed in Britain.

Extremists have to be dealt with. But, as Secretary Prior has said, the people of Northern Ireland who reject extremism have to be given more solid ground to work on. They need the kind of strength and responsibility promised by giving the new power-sharing plan an ungrudging, wholehearted try.