A few things here:
1) The Irish head of government is called the Taoiseach, not Prime Minister. And it's not a position you can simply run for in isolation: as in the UK, you need a majority of elected members of the legislature to endorse your candidacy. Unless McGregor is planning on founding a party and recruiting a good 100 or so willing candidates from across Ireland to run in the next Irish general election (which is 6 months away at most) then, no, it's functionally impossible for him to have a run at becoming Taoiseach.
2) What McGregor has actually suggested is that he might try to run for President of Ireland. But even here, there are a few obstacles. To get on the ballot, a presidential candidate has to be nominated by at least 4 local county or city councils. This might not sound like much, but there are only 31 such councils in total, and it's very likely that the (vast) majority of councillors will already be committed to backing their own party's candidate.
3) Even getting past all that, assuming McGregor did get on the ballot, and did win... the Irish presidency is largely a ceremonial position with very few powers, even compared to most presidents of parliamentary republics. There's very little of substance the president can do without the explicit approval or request of either the Taoiseach, the Dáil (the Irish parliament), or the Supreme Court. The closest British comparison isn't the Prime Minister, but the monarch - the Irish president exists to preside over ceremonial and diplomatic functions, not to run the country.