Three leaders of Irish-American organizations were arrested outside the British Embassy yesterday during a protest against British policy in Northern Ireland.

The protest, which drew about 40 people, coincided with the meeting at Camp David between British Prime Minister Margaret Thatcher and President Reagan. Organizers said the demonstration was patterned after protests against apartheid that have resulted in more than 70 arrrests recently outside the nearby South African embassy.

Arrested yesterday were the Rev. Sean McManus, 40, national director of the Irish National Caucus, a lobbying group that is based here; Joseph A. Roche, 49, national president of the Ancient Order of Hibernians, and Leo M. Cooney, 64, East Coast coordinator for the Irish National Caucus.

The three delivered a letter for Thatcher to an official on the steps of the embassy on Massachusetts Avenue NW. They were arrested when they refused a policeman's request to leave and were charged with demonstrating within 500 feet of an embassy, a misdemeanor.

Other demonstrators sang "Ireland Shall Be Free Someday" to the melody of the black American civil rights song, "We Shall Overcome."

Police said McManus and Roche were released without bail, and Cooney after posting $50 bond. All three are scheduled to appear Monday in D.C. Superior Court.

McManus, whose group organized the protest, said he was arrested at the antiapartheid demonstrations, where "I learned the technique." He said yesterday's demonstrators were seeking an end to what they called anti-Catholic discrimination in Northern Ireland. British officials were unavailible for comment.