The 37-year-old former health and higher education minister, best known for helping steer the country's initial response to COVID-19, was elected unopposed as the new leader of Fine Gael in March, just days after Varadkar's shock exit.
That all but assured he would become the 16th person to lead the country of 5.3 million, and he was confirmed in parliament after securing support from some independent MPs, as well as his Fianna Fail and Green Party coalition partners.
Harris will face the same deep-rooted problems, most notably a severe shortage of affordable housing and unease at record numbers of asylum seekers, that led to Fine Gael's stagnation under Varadkar, and inherits a coalition agreement that leaves little room for major new policy initiatives.
He is due to announce a reshuffle of his Fine Gael team - which makes up seven of the 18 seats in cabinet - on Tuesday.
It will not include the finance and foreign affairs portfolios, held by Fianna Fail's Michael McGrath and Micheal Martin.
Harris, who quit university aged 20 to work as a political aide, was elected to parliament at 24 and appointed to cabinet before he turned 30, used a speech at Saturday's Fine Gael annual conference to spell out his focus on law and order, helping small business and reconnecting with rural voters.
He also pledged to fix the housing crisis "once and for all" - something his predecessors have also promised - proposing an extension to support for developers and first-time buyers, while acknowledging the required boost to supply would take years.
Data on Monday showed asking prices for Irish homes rose by 6.5 per cent year-on-year in the first quarter, the fastest rate of growth since 2022.Â
An opinion poll on Sunday showed housing remained voters' top concern.
The same poll confirmed a recent trend of support for Sinn Fein dropping off highs of 12-18 months ago, though independent candidates rather than the government parties were again the main beneficiaries.
Sinn Fein stood on 26 per cent, with Fine Gael on 21 per cent and Fianna Fail on 16 per cent.
Other polls show support more closely aligned for the two main governing parties.
