The U.S. government is conducting a "substantial number" of investigations of suspected Irish terrorists and U.S. citizens involved in smuggling weapons and explosives to the Provisional Irish Republican Army and other related terrorist groups, according to federal law enforcement sources.

The sources said the investigations have taken on added urgency with the Oct. 13 IRA bombing of a hotel in Brighton, England, that narrowly missed killing British Prime Minister Margaret Thatcher and most of her Cabinet.

That bombing followed by two weeks the seizure by the Irish Navy of a trawler carrying seven tons of automatic weapons, ammunition and explosives from the United States to the IRA. Federal sources said the Irish government was tipped off by a U.S. intelligence agency about the shipment, the largest seizure of weapons there in the last 10 years.

Foreign and U.S. law enforcement sources said that the United States is the main source of weapons for the IRA and that funds raised by the New York-based Irish Northern Aid Committee (Noraid) are believed to be channeled largely into arms purchases.

After the seizure of the weapons off the Irish coast last month, Prime Minister Garret FitzGerald said Americans had sent the arms for Irish "subversives," and he denounced the willingness of U.S. citizens "to give aid for the purpose of sending arms and ammunition to murder people."

One FBI source said, "The perception in the British press is that there's much sympathy in the United States for the IRA and that we're dragging our feet. But that's just not true."

He added that under U.S. laws, the FBI can investigate only when terrorism or weapons trafficking is suspected. It is perfectly legal for U.S. citizens to contribute funds to Ireland for humanitarian and other purposes.

Michael Flannery, one of the founders of Noraid in 1970, has often said that his group was organized "for the sole purpose of feeding and clothing the homeless, the hungry and the naked," and has provided at least $3 million in assistance.

Flannery, 82, was indicted by U.S. authorities on gunrunning charges but was acquitted in 1982 after he said the CIA, rather than Noraid, was responsible for the operation and that he was simply following CIA instructions.

David Conklin, chief of the Firearms Division of the U.S. Bureau of Alcohol, Tobacco and Firearms, said that from 1970 through 1979 his agency investigated a large number of cases involving gunrunners for the IRA. He estimated that BATF handled eight to 10 major cases each year during that period, most of them in cities along the northeastern coast, particularly New York and Boston.

He said the volume of cases has dropped significantly since 1979, partly because of the arrests of a number of IRA leaders and partly in response to a growing new market for illegal arms in Latin America, both for political insurgents and for the illicit drug trade.

An increase in the number of bank robberies and kidnapings in Ireland also indicates that the flow of arms and funds from the United States may have decreased in recent years.

However, Conklin said there still are generally two or three major cases each year dealing with gunrunning for Irish terrorist groups. He added that there was a major case last year in which BATF agents posed as IRA gunrunners seeking to purchase machine guns. The agency eventually seized 99 MAC10 automatic weapons equipped with silencers, as well as an assassination kit. The 11 defendants were convicted and are awaiting sentencing.

Conklin would not discuss current investigations, but he said his agency cooperates closely with the Irish and British governments and is assisting in tracing the weapons seized from the trawler last month.

Other sources said that the FBI, which has primary responsibility in cases involving suspected terrorism, is "aggressively pursuing" a "substantial" number of investigations of "various individuals and groups" associated with terrorism in Northern Ireland.

A federal law enforcement source said those investigations included activities involving not only the IRA, but also the Irish National Liberation Army, an even more extreme left-wing organization. He added that the FBI is looking into the identities "of Irish terrorists allegedly operating in the United States."

Meanwhile, federal officials said a large ship, believed to have been the "mother ship" that loaded weapons just outside Irish waters onto the trawler intercepted last month by the Irish Navy, was seized by the Coast Guard last week in Boston harbor. The Boston office of the FBI is conducting an investigation of the ship's operations and its ownership.