Just to add to the very good info and correct a few bits of misinformation, as a parent of student about to graduate from TCD (waves to @ealingwestmum I've name changed and my kids would disown me if they suspected I ever post here!).
OP : have you fully read the CAO parents guide? It answers quite a few of your questions. Presumably your son has checked that he meets the matriculation requirements, particularly around modern European language, which catches out some people.
TCD engineering was 577* last year (and not ' maximum points'), same as at UCD.
UCD is generally been considered the more job-ready of the 2, for engineering.
UCD is definitely not the '2nd best Uni in Ireland' .....it's also an excellent Uni; its Smurfitt Business School is considered far better than TCD ( @Dublassie wow, your kids are stellar!) for starters!
I second considering TUD as another excellent job-market focussed engineering Uni, and also DCU, if the change of mind window is open.
I personally prefer the leafy UCD campus, its sports facilities are superb. I consider TCD is more like being at LSE, insofar as it is a city centre campus with lots of tourists by day!
UCD and TCD students mixing- variable. They are geographically several miles apart, but as a significant number are from Dublin, they will already know each other from school or home area (which is the only way mine has met people from UCD).
Mine is half-Irish, with Dublin family, and very used to visiting and holidaying there, but there are some small cultural norms and differences that took a bit of navigation at times. Just like you might have if you went to a different part of the UK I guess?
I'm not sure of the comparison with UCD, but TCD has a significant number of US students, who can bring their own flavour of expectations.. I've a different child at a UK Uni who has experienced the delightful ones as described by @NellyNoMates
Accommodation: Housing Crisis! Think London prices but without much supply.
Mine found Halls lonely at weekends, as most students went home to their non- Dublin counties every Friday, or if American, travelled overseas.
Mine did get 4th year campus accommodation randomly we assume, without being on any committees, societies, but those with those roles are apparently prioritised.
Cost of living, housing crisis: not to be underestimated! Food and going out are really expensive. Public transport is cheap though.
SUSI income thresholds are quiet low, and although they are grants (ie non-repayable) if you were to qualify, I would be concerned about how you would afford the rest of living in Ireland.
The Health system is very different, and you will miss the vastly superior NHS. Getting a GP required family connections and patience.
The deferral suggestion is a good one, but honestly, if they go, they'll probably fall in love with the place and may never come back!
Good luck to them!