The Taoiseach has urged the European Union to review trade ties with Israel over Israeli politicians' "despicable" ban of the UN's Palestinian aid agency, UNRWA.
Last night the Israeli parliament passed the bill to ban UNRWA activities with 92 votes in favour and ten against.
It came after years of harsh Israeli criticism of UNRWA, which has only increased since the start of the war in Gaza following Hamas's deadly 7 October attacks last year.
"The most important action that the European Union could take right now is reviewing trade relations," Mr Harris told reporters before meeting incoming European Council president Antonio Costa.
"What Israel and the Israeli Knesset did last night was despicable, disgraceful and shameful. More people will die, more children will starve," he said.
Mr Harris added there was "no alternative" to UNRWA, and that he would discuss with Mr Costa "how Europe now needs to find the moral courage... to act in relation to this.
"Ireland, Spain, Belgium, Slovenia and others have been calling for more actions at an EU level. I think that would be a very effective way and I'll be continuing to make that case," he said.
Mr Costa replaces outgoing EU Council chief Charles Michel on 1 December and is touring European capitals prior to taking up the new post.
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Earlier, an UNRWA spokesperson said the likelihood of the humanitarian operation in Gaza collapsing is "very, very high".
Speaking on RTÉ's Morning Ireland, UNRWA's Director of Communications Juliette Touma said the agency "did not have the right words" to describe the move, but called it "beyond concerning and beyond disappointing".
Ms Touma said the agency has no information on how or if the bill will be implemented.
At one point the agency was providing shelter to over a million people in Gaza, she said, and has been the largest provider of food and healthcare.
"We have the infrastructure ... it is very, very difficult to replace UNRWA," she said, adding that instead of focusing on UNRWA, Israel must focus on a ceasefire and peace.
Speaking on the same programme, Tánaiste and Minister for Foreign Affairs Micheál Martin said he condemned the move by Israel "unreservedly" and said "it will be impossible for UNRWA to carry out its life saving work unless something changes here".
Explainer: Who are UNRWA and what work do they do?
He said it represented the "further collective punishment of an entire population" that is already suffering the most extreme humanitarian crisis.
"This is an attack on Palestinians. What's happening in northern Gaza in particular is quite shocking."
Earlier, UN Secretary-General Antonio Guterres said the implementation of the law "could have devastating consequences for Palestine refugees in the Occupied Palestinian Territory, which is unacceptable."
"There is no alternative to UNRWA," he said in a statement.
"The implementation of these laws would be detrimental for the resolution of the Israeli-Palestinian conflict and for peace and security in the region as a whole.
"As I said before, UNRWA is indispensable."
Mr Guterres said he would bring the matter to the attention of the 193-member UN General Assembly.
The UN agency has provided essential aid and assistance across Palestinian territories and to Palestinian refugees elsewhere for more than seven decades.
UNRWA chief Philippe Lazzarini condemned the decision, saying it set "a dangerous precedent".
"This is the latest in the ongoing campaign to discredit UNRWA... These bills will only deepen the suffering of Palestinians," he wrote on social media.
The move also brought condemnation and criticism from the governments of the United States, Britain and Germany, among others.
Ahead of the vote, the US said it was "deeply concerned" about the bill, reiterating the "critical" role the agency plays in distributing humanitarian aid in Gaza.
Washington warned Israel on 15 October that it had 30 days to increase the amount of aid reaching Gaza or it would consider withholding some military assistance to its key ally.
UK Prime Minister Keir Starmer said Britain was "gravely concerned" as the legislation would make "UNRWA's essential work for Palestinians impossible, jeopardising the entire international humanitarian response in Gaza and delivery of essential health and education services in the West Bank".
In January, Israel accused a dozen of UNRWA's Gaza employees of involvement in the 7 October 2023 attack by Hamas, which sparked the deadliest war in the territory.
A series of probes found some "neutrality related issues" at UNRWA, and determined that nine employees "may have been involved" in the 7 October attack, but found no evidence for Israel's central allegations.
"There is a deep connection between the terrorist organisation (Hamas) and UNRWA, and Israel cannot put up with it," Yuli Edelstein, a politician from Benjamin Netanyahu's ruling Likud party and one of the sponsors of the bill, said in parliament as he presented the proposal.
"There is no place for enemies in the heart of the capital of the Jewish people."
Israel claims the whole of Jerusalem, including the annexed east, as its indivisible capital.
Mr Netanyahu said "UNRWA workers involved in terrorist activities against Israel must be held accountable" but added that "sustained humanitarian aid must remain available in Gaza".
"We stand ready to work with our international partners to ensure Israel continues to facilitate humanitarian aid to civilians in Gaza in a way that does not threaten Israel's security," he said on social media.
Islamist group Hamas reacted with fury to the ban.
"We consider this part of the Zionist war and aggression against our people," Hamas said in a statement.
The Palestinian presidency condemned the move and said it confirmed "Israel's transformation into a fascist state".
The German government "sharply" criticised the bill while the governments of Norway, Spain and Slovenia - which have all recognised a Palestinian state - issued a joint statement condemning the move.
World Health Organization chief Tedros Adhanom Ghebreyesus said UNRWA provided "an irreplaceable lifeline to the Palestinian people" and warned of the "devastating consequences" of implementing the bill.
"This is intolerable. It contravenes Israel's obligations and responsibilities, and threatens the lives and health of all those who depend on UNRWA," he wrote on social media.
The ban, which is due to come into force in 90 days, would effectively prevent UNRWA from operating in Israel and also target its operations in east Jerusalem, where it currently provides some essential services such as cleaning, education and healthcare in certain neighbourhoods.
It is also feared that UNRWA employees in the West Bank could potentially face problems moving from one place to another as well as accessing east Jerusalem or Israel because they would lose their ability to coordinate with the Israeli authorities to cross checkpoints.
The same fears apply to visas and permits delivered by Israeli authorities.
UNRWA and other humanitarian agencies have accused Israeli authorities of restricting aid flows into Gaza, where almost all of the territory's 2.4 million people have been displaced at least once during the war.
The agency itself has suffered heavy losses, with at least 223 of its staff killed and two-thirds of the agency's facilities in Gaza damaged or destroyed since the war began.
UNRWA was created in 1949 to support Palestinian refugees in the Middle East.
