I don't know who would be best out of the current selection, but I'm going on 'who would represent the nation well, or at least OK, at home and abroad?' because that's what the job is mostly about.
The Presidency isn't totally removed from politics, I know, but it is above party politics, so somebody with very their own rigid social or political opinions that they'd promote at every opportunity wouldn't be suitable. A good president is a president for all the people, not just for that section of the people who are as opinionated as them.
It's a tricky job, treading a careful line between being carefully diplomatic and being a wet rag, but they're well paid to do that!
Until there is a successful referendum to change it, Irish is the first national language, according to the Constitution the president is sworn to defend. Nobody is saying that they should be able to do in-depth interviews as Gaeilge or anything like that, but they should be able to make prepared speeches etc. in Irish.
Any candidate who has been considering a run for the Ćras has had ample time to get their Gaeilge up to a functional level, and there are many resources available online etc these days, so no excuse. Having a working ability to start with, and committing to learn more, as Mary McAleese did, would be OK.
But being a fluent Irish speaker is not enough, that wouldn't override the need for openness and diplomacy.
It shouldn't have to be said that a rapist would not be a good person to represent the nation at home or abroad, but yeah, it's that kind of campaignš¦
On a funny note, somebody suggested Liam Neeson in 'Taken' mode - if he was president, nobody would even think about messing with usš