In a darkened room at the officers' club here, three Irish soldiers on a midday break watch a hard-core pornographic movie on a large color television set attached to a video tape player.

Farther down the coastal road toward the nearby Israeli border, French, Dutch and other foreign soldiers swim, skin dive, sail or wind surf in the blue waters of the Mediterranean Sea.

While their recreational activities may be a bit more elaborate, the troops of the United Nations Interim Force in Lebanon (UNIFIL) undoubtedly behave during their off-duty hours much like the soldiers of the various warring armies elsewhere in the country.

Yet it is hard to escape the impression that the 7,000-man, international peace-keeping force, formed to police southern Lebanon after Israel's 1978 invasion, has very little to do now that the Israelis have invaded once again, reoccupied southern Lebanon and swept north all the way to the outskirts of Beirut.

The force's mandate, which was to end Thursday, was extended for another two months by the Security Council Tuesday, United Press International reported. The council, which met for three minutes, voted 13-0, with the Soviet Union and Poland abstaining.

The Israelis had objected to extending the mandate, but U.N. and Lebanese officials had argued that the force should be kept in place.

"At the moment we might be seen as a bit irrelevant, but if you pull out the U.N. now, you would be giving in to the situation completely," a U.N. official here said. "You would give away the last vestige of Lebanese sovereignty." He argued that with 800 Lebanese Army troops still attached to the peace-keeping force, "the only vestige of government authority in occupied Lebanon is in the UNIFIL area."

When the force arrived in April 1978, it was charged with verifying Israel's withdrawal following its invasion of southern Lebanon the previous month, establishing peace and security in the area and helping the Lebanese government reassert its authority in the south. Although it was never envisaged as a buffer force, that in effect is what it became.

Under its mandate, UNIFIL tried to keep "armed elements" out of its area but found itself caught between Palestinian guerrillas trying to infiltrate back into former positions from the north and the rightist, Christian-led militia of a renegade Lebanese Army major, Saad Haddad, who was supported by the Israelis in a border strip to the south.

Now the U.N. force no longer has to worry much about Palestinian guerrillas in its area south of the Litani River, and despite several feeble attempts, it was powerless to stop Israel's drive north, but it is trying to check the spread of new armed groups, U.N. officials said.

The main problem, one said, is a new Shiite Moslem militia set up by Israel independently of the forces under Haddad.

According to the official, "minor skirmishes" have resulted when UNIFIL troops have encountered the new militia, but there have been no serious incidents.

The majority of the inhabitants of southern Lebanon are Shiite Moslems, and the Israelis have been trying to curry favor with them along with the Lebanese Christians and Druzes -- apparently as part of a general policy to form links with Middle East minority gruops outside the Sunni Moslem mainstream.

UNIFIL officials point to their efforts to contain the new militia as an example of how they are still carrying out their original mandate. They also say UNIFIL troops have maintained all the positions they held before the Israeli invasion and have kept up their daily routine.

"The important thing is the fact that the mere presence of the U.N. is a big comfort to people in south Lebanon," said one UNIFIL officer.

A civilian U.N. official said, "We provide comfort and security to the people who live here because they know we are neutral."

While unable to prevent the Israeli onslaught in June, some UNIFIL units managed to slow it, U.N. officials said. When the Israelis first crossed the border on the coastal road, one official recalled, soldiers at a Dutch checkpoint set up antitank obstacles made of crossed iron beams, causing two tanks to ruin their tracks. However, the U.N. soldiers eventually ran out of the obstacles, the official explained.

Perhaps the most resolute attempt to block the Israeli invasion came at the Khardali Bridge over the Litani River where, the U.N. official said, eight Nepalese Gurkhas formed a human barrier and refused to move for two days and two nights. According to the official, the Israeli tanks eventually moved upstream and forded the river at a shallow point, then bulldozed the local Nepalese headquarters in revenge.

The civilian U.N. official said UNIFIL's responsibilities have expanded because of the large population increase in the south since the force arrived. In April 1978, about 1,000 persons were living in the UNIFIL zone, most of the inhabitants having fled the Israeli invaders. By June 6, the population had risen to about 250,000, the official said, and now it is about 350,000.

The influx since the invasion is attributed to the return of former refugees to their homes -- some had fled from Palestinian guerrillas -- and the arrival of new refugees fleeing the Israeli siege of Beirut.

Currently UNIFIL includes battalions from the Netherlands, Fiji, Ireland, Nigeria, Ghana, Norway, Nepal, France and Lebanon, a team of pilots from Italy and a Swedish medical unit. With an annual budget of $180 million, it is the largest and most expensive U.N. peace-keeping force in the world, outnumbering the 1,200 troops stationed on the Golan Heights and 2,300 on Cyprus.