Sinn Fein, the legal political arm of the outlawed Provisional Irish Republican Army, has made a significant first-time showing in local elections in Northern Ireland, a performance that could increase pressure on Britain for new political moves to resolve the disputes that plague the bitterly divided province.

The nearly final election results announced tonight for voting that took place on Wednesday immediately led to the establishment of an informal pact between Northern Ireland's two principal Protestant parties. Political analysts said the alliance would increase polarization in the province.

The Democratic Unionists, led by the Rev. Ian Paisley, and the Official Unionists, led by James Molyneaux -- together representing the province's two-thirds Protestant majority -- vowed that they would work to keep Sinn Fein from positions of power within Northern Ireland's 26 town councils. The two Unionist parties won 59.7 percent of Wednesday's vote.

The elections were the first local races in which Sinn Fein has participated. Although the party won only 58, or about 11 percent, of the 566 seats in the six counties of the north, it gained representation in 17 of the local councils.

Sinn Fein spokesman Danny Morrison said in Belfast that the vote "lay bare the British lie, which states that the resistance and struggle in Ireland has no popular mandate."

The results are seen here as a particular victory for Sinn Fein leader Gerry Adams, who argued -- against the IRA itself -- that the organization should have a local political, as well as military, role.

The British government had hoped that sentiment in favor of the IRA, which is seeking to drive the British from the north and unite the province with the predominantly Catholic Irish Republic to the south, had waned after the surge that followed the 1981 deaths of 10 jailed guerrilla hunger strikers. Sympathy for the hunger strikers translated into victory for a Sinn Fein candidate for the British Parliament.

Sinn Fein made notable gains in 1982 provincial assembly elections, while refusing to take the seats it won, but was outpolled by the moderate nationalist Social Democratic and Labor Party in last year's European Parliament voting. The party currently has one seat in the Parliament at Westminster, which it also refuses to take. It has pledged to participate in the town councils, however.

The SDLP, with 100 seats across the province, outpolled Sinn Fein again today. The moderate Alliance Party, which includes both Catholics and Protestants, placed last, with 34 seats.

Government officials in Dublin, who have been engaged for the past year in talks with London over the Northern Ireland problem, said they were not surprised at the vote.

Irish Foreign Minister Peter Barry said: "I think that the sense of alienation among the nationalists," meaning the Catholic community, "is again indicated by this vote."

Barry and others have expressed the fear that if Britain does nothing to try to ease the situation of Catholics in the north and move Ulster toward some kind of association with Ireland, the power of Sinn Fein will grow along with the prospects for greater violence. The British, however, are pledged to continued support of the majority view, and the Protestants want to retain British authority over the north.

The Dublin-London talks have been described here as a parallel "track" to British efforts to boost moderate Northern Irish political parties. Their goal has been to negotiate Dublin's participation in some nonsovereign affairs of the province, such as internal security, as a way of giving the Catholics a greater voice.

British government officials said today the elections would bring no change in their domestic Northern Ireland policy, including their refusal to talk to Sinn Fein.

The minister for Northern Ireland, Nicholas Scott, said the policy would continue until "when and if they renounce violence. Certainly not as long as they regard support for what they call the armed struggle as part of their policy will British ministers have anything to do with them."

The local councils have limited powers, since Britain took control of education and housing after civil strife erupted in 1969.

Although the nearly final tally showed Sinn Fein as a majority party on only one council, in Omagh, in Tyrone County, the presence of even one party member on any of the councils is likely to bring direct confrontation with the Unionists.

Paisley said today that "there will be no fraternizing with these godfathers of gunmen, bombers and murderers . . . "

The election is likely to cause more delicate, and potentially more serious problems for the SDLP. While politically opposed to the Unionists, the SDLP, which advocates a peaceful, negotiated solution to the Northern Irish crisis, also opposes Sinn Fein and the IRA.

"The SDLP could decide not to have any truck with Sinn Fein in the councils," a Dublin official said, with the result that "the Unionists would grab" key power positions, undermining the SDLP's position with its own voters and possibly boosting Sinn Fein's.

"On the other hand," he said, "what the SDLP is all about is responsible, moderate nationalism."

The SDLP did not rule today out the possibility of contacts to head off the Unionist alliance. Party chairman Alban Maginnis told reporters that "I think in certain circumstances we will have to consider talking to Sinn Fein and other political parties."