Leo Varadkar, the right-wing son of an immigrant Indian doctor, is set to be confirmed as Ireland's youngest and first gay premier during a vote in Dublin's parliament.
The 38-year-old will take over from Enda Kenny, who stood down on Tuesday to allow his successor as leader of the minority ruling Fine Gael party to also become Taoiseach.
In a drawn out heave to replace Mr Kenny, Mr Varadkar secured the backing of a majority of the party's elected representatives earlier this month.
His rival, 44-year-old father of three Simon Coveney, had the hearts and minds of most of the grassroots membership and has since been appointed deputy leader.
"Simon and I will work together to guide the participation by Fine Gael in the Government, and reform and modernise the Fine Gael Party in the years ahead," Mr Varadkar said.
Mr Varadkar, a qualified doctor, only revealed he was gay months before Ireland became the first country in the world to back same-sex marriage in a referendum in May 2015.
He has been in a relationship with another medic for about two years.
Mr Kenny announced last month he was stepping down after 15 years leading Fine Gael and six years as Taoiseach.
He had been under sustained pressure within his own ranks to allow a successor to be elected so the party is prepared for the next general election.
As Mr Varadkar prepares to take on the most powerful job in the country, questions already loom over how long he can hold on to the office.
The administration is propped up by Fine Gael's ancient rivals Fianna Fail as part of a supply and confidence arrangement after an unprecedented schism in the electorate.
During a farewell address to the Dail, Mr Kenny said he hopes his legacy will be marked by a modest contribution to improving the country.
In a brief and modest valedictory speech, he said he accepted he had made mistakes but that he always had people's best interests at heart.
"This has never been about me... it has always been about the problems and challenges that the people of our country face," he said.
As part of his last day in office, the veteran Fine Gael politician hosted a Cabinet meeting for the final time in Government Buildings before calling on President Michael D Higgins with a formal letter of resignation.
On his way to Aras an Uachtarain, he stopped off in Dublin's north inner city - an area blighted by high unemployment and gangland violence.