Foreign Affairs Minister Charlie Flanagan has warned the Israeli ambassador that the shelling of a United Nations run school in Gaza breaks international law.
Amid anger from backbench TDs over the Government's refusal to support a United Nations resolution on alleged war crimes in the Palestinian enclave, Mr Flanagan held a second round of talks in a week with Boaz Modai.
The minister told the ambassador he condemned the shelling of the school where 15 people died as they sheltered.
And he said he reiterated the very grave concerns of the Irish Government and Irish people at the situation in Gaza and Israel.
"I expressed my own deep disquiet and the grave concerns of the Irish Government and people at the escalation of violence in Gaza and at yesterday's latest atrocity which saw a UN-run school hit and at least 15 civilians killed and more than 200 wounded, including many children," Mr Flanagan said.
"Such attacks on civilian institutions such as schools and hospitals are utterly unacceptable, contrary to international law and must stop immediately."
Mr Flanagan was criticised by Labour backbenchers over Ireland's stance at the UN Human Rights Council in Geneva on Wednesday evening.
The Government refused to support a resolution seeking an investigation into alleged war crime by Israel claiming the motion did not recognise the indiscriminate rocket fire from Hamas and other militants in Gaza.
The mostly civilian death toll, mainly in Gaza, has risen to more than 800.
Mr Flanagan issued a statement on the talks with the ambassador as talks intensified on a ceasefire with US secretary of state John Kerry leading calls to nail down an agreement.
"I again reiterated to the ambassador that the appalling level of civilian casualties which we are now seeing in Gaza as a result of Israel's current military operation cannot be justified under any circumstances," the minister said.
"The violence on all sides must stop now and an immediate ceasefire be agreed."
The ambassador said he would inform his government of the Irish position.
Sinn Fein leader Gerry Adams has called for the Dail to be recalled to discuss the humanitarian crisis in Gaza and the response of the Irish Government.