A window in Northern Ireland

Is a window opening for political progress in Northern Ireland? It may be, says James Prior, Britain's secretary of state for Northern Ireland, and it would be ''a deeply tragic mistake'' to let any opportunity slip. Now we hear of a poll among the Northern Irish that may lend weight to his hopes - and to his efforts for an initiative away from Britain's present direct rule of the province. The poll by Ulster Television found that two-thirds of the Roman Catholic minority as well as three-quarters of the Protestant majority want to remain part of the United Kingdom under a power-sharing government of their own.

Power sharing between Northern Ireland's factions has been tried before. Such efforts face internal dissension, of course, as well as the long-range aspirations of some for breaking with Britain and uniting with the Republic of Ireland. The republic's Prime Minister Haughey spoke to such aspirations on his recent return to office after Garret FitzGerald's brief tenure. He even sought to enlist US President Reagan in the cause.

But no one seriously imagines such an outcome without the consent of the people of Northern Ireland. The recent poll suggests the time is not now.

Which leaves Mr. Prior's possibly opening window for something short of a united Ireland. ''We have to face the fact that we cannot make direct rule work indefinitely,'' he said last month. He said that to give up the chance of trying to restore some greater power and responsibility back to the politicians ''would be to resign ourselves to a very unsatisfactory security situation.'' The worst and most likely result, he said, would be that security would steadily deteriorate because of prolonged uncertainty. Nor was there much hope of establishing a sound economic base in Northern Ireland with the political uncertainty that goes with direct rule.

To carry on indefinitely as at present, he warned, would mean that the more moderate political figures would never come to the fore. ''For them to operate effectively and show their supporters that they are worth supporting, they have to be able to play their part in democratic and acceptable institutions.''

Mr. Prior sees that, whatever the political result, it cannot be imposed by Britain. He would try rather to create conditions enabling the parties and politicians in Northern Ireland themselves to hammer out goals and ''move towards a more settled and stable future.''

Mr. Prior is understandably cautious, testing the waters. But he is right not to let any opening window go unnoticed.