London — A new attempt is being prepared by Prime Minister Margaret Thatcher to restore a limited form of self-rule to Northern Ireland.
A plan drawn up by the British prime minister's Ulster secretary, James Prior , proposes an assembly with elections to be held this autumn and an executive council in Belfast on which Protestants and Roman Catholics would share power.
Mr. Prior's initiative has yet to be approved by the Thatcher Cabinet, and when the moment for launching the plan arrives (probably in the spring) great care will be taken to prepare Northern Ireland opinion.
The idea of an assembly is strongly favored by many British members of Parliament who see Britain's position in Ulster bogging down hopelessly if fresh efforts are not made to devolve power to the troubled province.
Since the early 1970s, Ulster has been ruled directly from London. All executive decisions affecting the province are taken by Mr. Prior who came to office as Ulster secretary late last year.
An attempt at power-sharing between the Protestant majority and the Roman Catholic minority came to grief nearly 10 years ago when the Protestants refused to cooperate. This failure was followed by a steady escalation of violence.
Mr. Prior is launching his plan at a time when shootings and bombings have been occurring at only a moderate level. The British government feels that the atmosphere is about right for another initiative.
Mrs. Thatcher is hoping that the general election in Ireland later this month will produce a stable majority in the Dublin Parliament and that the new government, whatever its political complexion, will support the Ulster initiative.
The plan Mr. Prior has devised would produce an assembly of about 80 members. Initially it would have few powers. The Protestant Unionists would inevitably have a majority.
But Mrs. Thatcher's Northern Ireland secretary wants to introduce a shrewd ploy: the assembly will not be granted executive powers unless and until Protestants and Catholics reach agreement on what powers the executive should have and which assembly members should exercise them.
If an agreement can be struck, Britain would then act to set up an Ulster executive to absorb at least some of the powers currently wielded by the Government in London.
Mr. Prior is counting on the fact that virtually everyone in Ulster's political world wants power to be returned to Belfast. In effect he is saying ''you can have power when you agree an equitable way of using it.''
Government officials at Whitehall who have been quietly releasing advance details of Prior's plan accept that problems lie ahead. All the political parties in Ulster are suspicious.
The Rev. Ian Paisley, leader of the Democratic Unionist Party, is likely to demand a veto over any decisions the assembly makes.
Mr. Prior apparently hopes to outflank Mr. Paisley by enabling the official Unionists and the mainly Catholic Social Democratic and Labour Parties to contest free elections. Some analysts currently detect a slippage of popular support for Mr. Paisley in recent months.
Both in Whitehall and at Mr. Prior's Belfast headquarters, unrestrained optimism is being strongly discouraged. The British government could fail in its pursuit of a power-sharing solution.
Mr. Prior wants to seize the right moment for a possible solution. The full British Cabinet is likely to be asked to give its backing to the power-sharing plan in early March.