The commander of the Christian militia in southern Lebanon today demanded that the 600-man Irish battalion in the United Nations peacekeeping force leave Lebanon because it has become sympathetic to the Palestine Liberation Organization.

Maj. Saad Haddad, the leader of the Israeli-supported Free Lebanon enclave, said the Irish peacekeeping unit has provided tacit assistance to PLO guerrillas ever since Irish Foreign Minister Brian Lenihan said on Feb. 10 in Bahrain that Palestinians have a right to self-determination.

"The Irish are helping the terrorists. They are receiving orders from their country to help the PLO because they made an agreement to recognize the PLO," Haddad said in an interview in this border town.

"It is better if they get out of Lebanon. It will be better for everyone," said Haddad, whose several hundred regulars and 500 militiamen control the secessionist enclave stretching five miles deep for 60 miles along the border from the Mediterranean to the foothills of Mount Hermon.

U.N. officials denied Haddad's charges, and said the militia had forced its way into areas assigned to U.N. control.

Tensions between Haddad's forces and the Irish battalion have been high since the militia rammed its way into the village of At Tiri in the Irish zone last Sunday and demanded the right to establish a position there. When Irish troops, backed by reinforcements from Dutch and Ghanaian units, encircled At Tiri, the militia opened fire with machine guns and captured nine Irish soldiers, holding them for several days. One Irish soldier is in critical condition in an Israeli hospital.

Ireland's nonresident ambassador to Israel, Sean Ronan, arrived here this week to talk with Israeli officials about Haddad's militia. While Israel insists it has no control over the militia, Israeli liaison officers are constantly in the enclave and Haddad regularly appears in Metulla for meetings with high-ranking Israeli officers.

The Israeli Army supplies and pays Haddad's forces.

Haddad today said his troops will refuse to leave At Tiri, and he charged the village is along a route through which Palestinian guerrillas regularly pass.

"The people of At Tiri came and asked for our help. They . . . know the U.N. cannot stop the infiltration of terrorists," said Haddad, who derisively calls the Irish troops "Johnny Walkers."

Haddad's allegations, even taking into account his explosive and quixotic personality, underscores the difficulties of trying to achieve a semblance of stability in southern Lebanon amid so many hostile factions.

Israel's incursion into Lebanon Wednesday to establish a line of "defensive positions" stretching 12 miles between the villages of Markabe and Kunin has complicated that process, according to the United Nations, which has lodged a protest with Israel.

Almost every day for the past several weeks, U.N. forces have been fired upon either by Haddad's forces or by Palestinian guerrillas, whom Haddad and the Israelis say roam freely in the U.N. zone that is sandwiched between the Christian enclave and the Litani River to the north. Israel claims that 700 guerrillas, backed by 1,000 armed leftist supporters, are in southern Lebanon.

U.N. soldiers passing in and out of this border town complained they cannot begin to keep peace in southern Lebanon as long as Haddad's militiamen, who are ill-disciplined in the use of their weapons encroach on the U.N. zone and fire indiscriminately at guerrillas and U.N. posts alike.

The Israelis say they have no confidence that the U.N. troops can intercept Palestinian infiltrators. They complain that it was through the Irish zone that a terrorist squad passed on Monday on its way to the Misgav Am border kibbutz, where three Israelis were killed.

The Israeli Army incursion Wednesday was, in effect, also an expression of no confidence in Haddad's forces. Some of the 350 Israeli troops the United Nations says are in Lebanon are assisting the militia patrol the Haddad salient.

The Israeli Army, following its usual practice, has refused to discuss details of its incursion, the first since last May. But Israeli officers said privately, however, that the purpose was to establish observation posts because intelligence had indicated that the PLO plans terrorist operations inside Israel during Prime Minister Menachem Begin's trip to Washington next week.

Yet there were signs here today that the Israelis plan to stay in Lebanon for a while. The Army refused to allow reporters near the new positions three miles inside Lebanon, but U.N. officers said the Army has dug well-fortified positions with bulldozers and pitched tents at the three positions.