Ireland has pledged three million euro in humanitarian funding to help refugees fleeing Syria.
"Our contribution today recognises the unprecedented scale of the crisis and the enormous demands on humanitarian agencies and neighbouring states," he said.
"This funding will benefit those directly affected by the conflict and the communities here in Lebanon and in other neighbouring states which have shown such extraordinary generosity and solidarity to those fleeing Syria."
The camp was established in 1948 to house Palestinian refugees from Galilee but has more recently become home to Palestinian refugees fleeing the conflict in Syria.
Mr Costello saw at first hand the relief efforts at a health centre and a school in the camp during his visit.
Ireland has donated almost 14 million euro to the international humanitarian response to the conflict, with one million euro of the latest aid to be directed to UN agencies in Lebanon.
'The UN High Commissioner for Refugees has described the conflict in Syria as the worst humanitarian disaster since the end of the Cold War," Mr Costello added.
"Almost seven million people require urgent assistance to meet their basic needs and more than two million people have fled to neighbouring countries.
"Lebanon is host to almost three quarter of a million refugees from Syria in addition to over a quarter of a million Palestinian refugees.
"They struggle to access adequate shelter and basic services and many are vulnerable to violence and sexual exploitation."