But you're fighting a straw man here. It's not about every Irish passport holder getting a vote - that would be daft and I haven't heard anyone seriously proposing it.
There are apparently 300-400 thousand Irish nationals who left the state in the last 10 years (some returned, some left again, hence the range). For reference, the Irish electorate is around 3.3 million.
Letting expats vote in presidential elections? Not a big deal. Since the president is the international figurehead of the state, expats are affected by who holds the role. I think they deserve a vote on it.
Creating a special expat constituency with say 2-3 TDs for Dáil elections? I could get behind that. Many expats own property (or mortgages in negative equity, rather), hold pensions, have savings, etc. in the state so they are affected by Irish taxation and other policies. A separate constituency capped at small number of TDs would give them representation, but proportionately less than residents.
And if a particular expat feels they shouldn't have a say because they've no ties to Ireland any more... then don't vote. Same as any electorate, not everybody bothers.
Basically, I can see lots of acceptable ways it could work so it doesn't make sense to rule it out before seeing the specific proposal.