Two Irish citizens were charged today in Dublin's special antiterrorist court with the bombing murder Monday of Lord Louis Mountbatten.

Police said the two men, Francis McGirl, 24, and Thomas McMahon, 32, both citizens of the Irish Republic, first were arrested two hours before Mountbatten's yacht was bombed Monday when an Irish police officer became suspicious during a routine road check about 80 miles south of the assassination scene.

Irish police conduct such random checks for possible terrorists as part of their normal duties, officials in Dublin said.

Although Irish officials were jubilant at the possible breakthrough, they made it clear that their investigation has just begun. Police said they raided several homes of suspected members of the outlawed Irish Republican Army last night as part of the biggest antiterrorist operation in the history of the Irish Republic.

Meanwhile, Irish Prime Minister Jack Lynch accepted invitations to meet with British Prime Minister Margaret Thatcher and to attend Lord Mountbatten's funeral Wednesday.

After a Cabinet meeting today, the British government announced that it would increase the strength of the Royal Ulster Constabulary by 1,000, a move seen as part of a continuing British effort to shift more of the responsibility for Ulster security from Army units to the police.

The two suspects charged today were raced to the special court in Dublin in an Army convoy. As soldiers guarded the courtroom, the judge ordered McGirl and McMahon held until Oct. 2. The entire procedure lasted 4 1/2 minutes.

The two men were stopped Monday at 9:30 near the town of Granard in County Leitrim. The arresting officer stopped the car on a hunch, according to a colleague, and noticed that the driver, McGirl, was "shaking like a leaf."

McGirl gave police a false name and address, but when police discovered his identity, their suspicions were further aroused, as McGirl and his family had been suspected of IRA involvement before, police officials said.

About three hours after arresting the two men, Irish police said they heard of the bombing that killed Lord Mountbatten and three other persons. McGirl and McMahon then were questioned closely about possible IRA contacts, but it took almost 24 hours before police determined that they might be suspects in the bombing.

McGirl and McMahon subsequently were charged with being members of the IRA, but were released on a technicality. They were immediately rearrested by Irish detectives and charged again, this time successfully.

Yesterday, Irish officials said the two suspects would appear in court again today to face "explosives charges." According to unconfirmed reports, police had found traces of nitroglycerine on the men's clothing. The two were charged with Lord Mountbatten's murder when they appeared to face the explosives charges.

McGirl and McMahon are farmers who live near the scene of their arrest. McGirl worked on a small farm with his parents, and local residents say he spent most of his time driving a tractor or helping neighboring farmers.

Neither man has made a public statement since his arrest, and both remained silent as they were charged with murder this morning.

Meanwhile, investigators are trying to reassemble as much of Lord Mountbatten's boat as possible. Police divers have continued to pull up pieces of the wrecked vessel in an effort to determine the type of bomb used and how it was detonated.

Police have not ruled out that the bomb could have been exploded by remote control, as the IRA claims. If that is the case, others would have to have been involved, since McGirl and McMahon were arrested two hours before the bomb was detonated.

Irish police are not ruling out further arrests.

The Irish government has offered a $220,000 reward for information leading to the arrest and conviction of those responsible for killing Lord Mountbatten and members of his family. British officials, however, are not overly optimistic that the reward will bring results. A similar reward for $110,000, offered after 1976 assassination of the British ambassador to Ireland, is still uncollected.

Prime Minister Lynch flew home today from a vacation in Portugal in time to see off the coffins bearing the bodies of Lord Mountbatten and the two slain members of his family in a full military ceremony at Finnear Airbase in the Irish Republic.

The Royal Air Force plane bearing Lord Mountbatten's flag-draped coffin landed in Southampton, England, where it was met by an RAF escort, Prince Philip and Prince Charles.

The bodies were taken to lie for a day in Romsey Abbey near Lord Mountbatten's home in Hampshire. On Tuesday Lord Mountbatten's body will be taken to St. James's Palace in London, where it will lie in state until Wednesday's funeral at Westminster Abbey.

As tension in Belfast mounted, clashes between gangs of Protestant and Catholic youths at a soccer match south of Belfast left more than 100 persons injured. With feelings running so high, Scotland Yard police are worried about security for Lord Mountbatten's funeral, which is expected to bring foreign dignitaries from around the world.