UN spokesman Fred Eckhard said on Friday that Peter Fitzgerald of the Irish national police force, known as the Gardia Siochana, is expected to leave for Beirut in the next few days to begin work.
The UN Security Council had asked Secretary-General Kofi Annan to report urgently "on the circumstances, causes and consequences" of the assassination of al-Hariri in a bomb blast in Beirut on Monday.
The Bush administration wants Security Council members to consider measures that could be taken against al-Hariri's killers but it was unclear how many council members would agree.
France and the United States in September engineered a council resolution demanding Syrian troops get out of Lebanon. The measure squeaked through 9-0 with six abstentions. A minimum of nine votes is needed for a resolution to pass.
Lebanese are divided over whether Syria, which has more than 14,000 troops and police in the country, had a role in the assassination.