Simon Harris became Ireland's prime minister in waiting on Sunday after he was elected unopposed to succeed Leo Varadkar as leader of the governing Fine Gael party.

Harris, a 37-year-old minister best known for helping steer the country's initial response to COVID-19, will set out some of his priorities to party members at an event in the Irish midlands town of Athlone on Sunday.

He will then be voted in as Ireland's youngest ever prime minister when parliament next sits on April 9 thanks to support from Fine Gael's coalition partners.

Harris will have no more than a year to save the coalition from defeat at parliamentary elections. Polls for the last three years have put Sinn Fein, a left-wing party that backs unification with Northern Ireland, a British province, as the favourite to head the next government.

However, two more polls on Sunday confirmed a recent trend of support for Sinn Fein dropping off highs of 12-18 months ago, though they again broadly showed smaller parties and independent candidates as the beneficiaries over the government parties.

A Business Post/Red C poll conducted before Varadkar's exit put Sinn Fein's lead over a stalling Fine Gael at 6 percentage points, while an Irish Independent/Ireland Thinks survey after he quit showed a 5-point edge after a small rise for Fine Gael.

Varadkar announced his departure to widespread shock on Wednesday, catching even his closest political allies by surprise, saying Fine Gael would stand a better chance of re-election under another leader.

Harris has spoken in recent days of how he became involved in politics as an "opinionated, moody teenager" annoyed at the lack of educational supports for his autistic brother. He has sought to paint himself as an "accidental politician", even though he has spent most of his adult life in parliament.

He is one of Ireland's most visible government ministers and a strong media performer. His keen social media presence led one opponent in parliament to dub Harris the "TikTok taoiseach" (Irish for prime minister).

While the economy grew strongly under Varadkar, successive governments, of which Harris has been part, have struggled to tackle a decade-long housing crisis and, more recently, the pressure from record numbers of asylum seekers and refugees.

Inheriting a three-party coalition government working off an agreed policy programme will give Harris little room for any major new policy initiatives.