President Reagan formally proposed a five-year, $250 million aid program for Northern Ireland yesterday, and House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) urged Congress to support the plan as "a promise of peace for the people" of that strife-torn land.

The initiative, submitted to Congress two weeks before St. Patrick's Day, caused O'Neill to take the unusual step of appearing as a witness before the House Foreign Affairs subcommittee on Europe and the Middle East to give the proposal an emotional personal endorsement.

While the hearing made clear that the idea has overwhelming congressional support, comments from witnesses and committee members also revealed sharp differences about the nature of the aid package and conditions under which funds would be disbursed.

The proposal is intended as a gesture of support for the agreement last Nov. 15 between Britain, which has ruled Northern Island since the island's partition 64 years ago, and the Republic of Ireland. The accord would give Dublin an official voice in governing Ulster, as the province historically has been known.

The aim is to end 16 years of sectarian violence between Northern Ireland's majority Protestant and minority Roman Catholic communities by providing greater economic opportunities and protections for the rights of the minority. Part of the plan calls for creation of an international fund to administer development of the economically depressed province.

The administration seeks a U.S. contribution of $50 million a year over five years to aid this development in Northern Ireland and affected areas of the republic.

However, the administration proposal, as outlined for the 1986 and 1987 fiscal years, envisions disbursing $20 million in direct economic aid to the international fund and making available the remaining $30 million in the form of incentives to private business through investment guarantees and trade promotion programs.

In outlining the plan, Rozanne L. Ridgway, assistant secretary of state for European affairs, said budgetary restraints make it very difficult to provide more than $20 million in direct aid without taking funds from other programs.

However, O'Neill, who called $50 million a year "a reasonable sum," stressed that he believes that the aid should be "substantially in the nature of economic support funds and not loan guarantees."

Some members of Congress and spokesmen for various Irish-American groups contend that the United States should attach conditions to the aid to ensure fair distribution among Catholics and Protestants, not diversion for police or intelligence purposes.

That idea was opposed by administration witnesses and O'Neill, who said:

"I believe that Congress should make the aid subject to standard conditions for assistance and not make it contingent on a long list of political conditions. We need to express our unqualified support for the accord, not our doubts about it."