The three-member team of Irish police officers led by deputy commissioner Peter Fitzgerald was met at Beirut airport on Thursday by foreign ministry officials and the UN spokesman in Lebanon, Nejib Friji.
The team is due to "assist and cooperate" with Lebanese authorities and is expected to contact security and judicial officials in the country, according to sources close to the inquiry.
After a weekly cabinet session on Thursday, Information Minister Elie Firzli reiterated a pledge that the "Lebanese judiciary authorities will not hesitate to assist, cooperate and coordinate" with the team.
The UN team is made up of forensic, judicial and political experts.
Lebanese opposition parties have accused the government and its political masters in Syria of having a hand in the 14 February massive bomb blast in a Beirut seafront neighbourhood that killed al-Hariri and 17 others.
Kofi Annan says Syria must leave
Lebanon before April
The authorities in Beirut have denied any responsibility for the killing and agreed to cooperate with the UN commission of inquiry, but have rejected a full international probe.
The UN team is due to complete their initial investigation within a month.
Meanwhile, UN chief Kofi Annan has called for a Syrian troop pullout from Lebanon before April.
In an interview with Al-Arabiya news channel on Thursday, he warned that the Security Council could otherwise take "measures" against Damascus.