The Department of Foreign Affairs has urged all Irish citizens in the Libyan city of Benghazi to leave immediately, and has advised against all travel there.
Britain, Germany and the Netherlands all issued similar advice to their citizens this afternoon.
The Department said intelligence reports suggest that there is a specific, imminent threat to westerners in Benghazi.
Irish citizens, either visiting or living in Libya, are strongly advised to register their details with the Irish embassy in Rome, which can be done through the Department's website.
The move comes after a deadly attack by Islamist militants in neighbouring Algeria last weekend.
At least 38 hostages were killed in an attack on the remote In Amenas gas complex in Algeria, about 100 km from the Libyan border. French forces are also fighting Islamist rebels in Mali.
Britain's Foreign Office has warned in the past of the long reach of al-Qaeda in the Islamic Maghreb, the North African wing of al-Qaeda.
Few Westerners are believed to be in Benghazi, which has experienced a wave of violence targeting foreign diplomats, military and police officers, including an attack in September that killed the US ambassador and three other Americans.
Last week Italy suspended activity at its Benghazi consulate and withdrew staff after a gun attack on its consul.
"The situation in Cyrenaica (eastern Libya) is not just worrying, it is incredibly worrying. Everybody is on alert," a Western diplomat said. "But in light of the events recently (in Algeria and Mali), this could be a precautionary measure."
Saad al-Saitim, deputy head of the Benghazi Local Council, said the warning was a setback, inciting "more fear at a time when people need to stand with us".
British Airways said it would continue operating flights to the Libyan capital Tripoli. The airline operates three flights a week between London's Heathrow airport and Tripoli. Its next flight to Libya is scheduled for Sunday.