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Simon Harris said he would continue to work towards a ceasefire in Gaza as he became Ireland's new Taoiseach after politicians backed his nomination.
The Dail, Ireland's lower house, voted 88 to 69 in support of Mr Harris, who on Tuesday travelled to the residence of President Michael D Higgins where his appointment was confirmed.
Mr Harris, 37, who becomes Ireland's youngest Taoiseach, replaced Leo Varadkar as Fine Gael leader following the latter's surprise resignation last month.
In his acceptance speech, Mr Harris paid tribute to his predecessor, calling him "a trailblazer".
He promised "to be a Taoiseach for all" and work to "improve the lives of all in this country".
“Today I accept this new role in a spirit of humility, ready for the challenge, and full of energy and determination about what can be achieved,” he said on Tuesday.
“As Taoiseach I want to bring new ideas, a new energy and a new empathy to public life. But politics is not about the office-holder. This is not about me, it’s about all of us working together to serve the people."
He said his government would continue pressing for a ceasefire in Gaza.
Ireland has been critical of Israel's war campaign, with Foreign Minister Micheal Martin previously saying the Israeli military's conduct since October 7 "represents the blatant violation of international humanitarian law on a mass scale".
In February, Ireland said it would join South Africa's case against Israel in the International Court of Justice by attempting to widen the definition of genocide to include blocking the delivery of aid.
Mr Harris said: "In Gaza, we are witnessing humanitarian catastrophe. Men, women and children are being slaughtered and starved.
"We are not silent on Hamas's October 7 massacre but nor can we be silent on Israel's actions.
"As a country we will do our part to bring about a ceasefire and lasting peace."
Simon Harris becomes Irish Prime Minister - in pictures
During a debate ahead of the vote in the Dail, there were calls for Ireland to take a stronger line against Israel and introduce political measures that would put greater pressure on the Israeli government.
Labour leader Ivana Bacik said Irish government "needs to go beyond rhetoric", which she said only emboldened Prime Minister Benjamin Netanyahu.
"This government needs to act with Spain and recognise Palestinian statehood," she said, as she demanded "deeds not words".
"You need to pass the Occupied Territories Bill with the support of the opposition. You need to impose meaningful sanctions on the Israeli government ... that could bring about the end to suffering in Gaza.
Mr Harris, who was elected to Ireland's Parliament in 2011, has held a number of Cabinet jobs during his career in politics, including health minister during the Covid-19 pandemic.
Mr Harris's prominence on social media has made him one of the most visible politicians in Ireland.
The new Taoiseach will face a formidable to-do list, including tackling housing and homelessness crises amid criticism of government policy on asylum seekers.
With a reputation for slick communication skills, Mr Harris is expected to urgently seek to galvanise his struggling party, which trails in polls as key elections loom.
Mr Harris has assumed the lead of the Fine Gael, Fianna Fail and Green Party coalition government with less than a year to run before a general election must be called.
He said he hoped to lead a “partnership government” in a spirit of “unity, collaboration and mutual respect”.
Sinn Fein leader Mary Lou McDonald and her party voted against the appointment of Mr Harris. Ms McDonald has demanded a general election and accused the government of "clinging to power at all costs".
Earlier in the session, Mr Varadkar spoke in the Dail where he defended his record in office, particularly in regards to Ireland's recovering economy.
He said his time in government had been the “most fulfilling and rewarding time” of his life.
Mr Harris possessed "empathy, energy and campaigning skills", his predecessor added.
Ireland votes in local and European Parliament ballots on June 7, while the next general election must be held by March next year.
Fine Gael slumped to third place at the last general election in 2020, well behind leftist-nationalists Sinn Fein, the former political wing of the paramilitary IRA, which secured the largest share of the vote.
