The Taoiseach said Israel has crossed "yet another line" by deliberately firing on United Nations peacekeepers in Lebanon.
Simon Harris raised concerns over the deteriorating situation in a phone call with Israel's President Isaac Herzog earlier.
Mr Harris said he made clear the deliberate firing at UNIFIL posts is a breach of international law and a cause of deep concern to the Irish people.
It comes after a number of incidents in recent days involving the Israeli Defence Forces and United Nations peacekeepers.
At around 4:30 a.m., while peacekeepers were in shelters, two IDF Merkava tanks destroyed the position’s main gate and forcibly entered the position. They requested multiple times that the base turn out its lights.
— UNIFIL (@UNIFIL_) October 13, 2024
Taoiseach Simon Harris said he put his concerns directly to President Herzog.
"I took the opportunity to speak to the President of Israel today because I wanted to convey to him in the strongest possible terms the absolute importance of peacekeepers being respected and international law being upheld," he said.
"I conveyed to him my view that attacks on peacekeepers is not just utterly unacceptable but a clear breach of international law.
"In my view Israel has breached international law and I said that directly to the President".
'Actions and not words'
Mr Harris said he fully respects the right of any state to defend itself "but our peacekeepers deserve to be protected in any such scenario".
"Any such scenario to defend yourself also has to be proportionate - in my view Israel has crossed yet another line," he said.
"The first was crossed in Gaza with the brutal humanitarian impact on children and civilians and now we see a further line crossed in relation to peacekeepers - in the words of the United Nations - being deliberately fired on".
Mr Harris said President Herzog assured him that "Israel would take this matter extraordinary seriously".
"But being very honest, I will judge Israel on their actions and not their words in relation to this".
Mr Harris will attend an EU and Gulf Cooperation Council (GCC) summit in Brussels on Wednesday, where he will also speak with leaders of other countries involved in UNIFIL.
