A further 200 Irish peacekeeping troops will be deployed to Lebanon.
It is the second of two contingents bringing the total number of Irish soldiers with the United Nations-backed mission to 440.
Patrolling an area of 87 square miles, the soldiers have been sent to protect civilians and ensure humanitarian supplies reach them, as well as help Lebanon's armed forces.
Some 200 troops from the 104th Battalion were deployed on Thursday and an advance party of 90 specialist personnel has been in the country since the end of May. Fifty of those are returning home.
They were preparing camp and infrastructure for the battalion in the southern village of Tibnin in Bint Jbeil, and two smaller posts along the so-called blue line with Israel.
The youngest soldier of the group is 20-year-old Private Emma Furlong who grew up at the Curragh where her father Daniel is still based.
The current strength of the Unifil force is 12,000 personnel drawn from a total of 31 countries.
Irish officers were first sent to Lebanon, as observers, in 1958. The first battalion was there from 1978, as part of Unifil, when Israel invaded the country.
Since then 47 Irish troops have been killed in Lebanon.
The main Defence Forces battalion was withdrawn in 2001 after 23 years but was deployed again in 2006 for 12 months. The current mission is expected to last as long as four years.