Fabulous @5starzz, congratulations to your boy and especially well done on securing Trinity Halls, that's a huge weight off and will give him an great network of YP!
Mine is returning for her 4th year in Sept, she's been lucky enough to get accommodation on campus, the downside will be constant tourists passing her window but a minor inconvenience, and has saved us a fair bit in £.
She didn't open an Irish account so can't help there, she's worked from her 'home' account (Barclays) and just transfers to her Monzo/Chase when needed to pay in EUR, and Revolut between student transfer type stuff. I pay her fees either by EBT or via the card link, I believe the portal allows the accommodation to be paid monthly, it's slightly cheaper if you opt to pay in EUR vs GBP, or you get your son to lodge his payment details each time for monthly and you transfer to him. It's a bit clunky but still.
Re PT work, lots of the UK kids did secure bar/waiting tables type work, but for mine, the tutoring carried her through 3 years financing all but her fees/accom. And she travelled a lot. Only this year she is reducing teaching hours to near zero as she has saved enough adding summer internship monies to focus on the final year stuff.
My final tip for your boy which understandably will not be front of mind right now is to keep an eye on the spring week/summer internships/research grants etc timelines that are available for many on the island of Ireland (inc Belfast). Most of DD's peers applied successfully, given the type of corporations out there from an EU base, it seems just more proactive vs UK on that front but that is anecdotal feedback from what we've observed.
Very best of luck to him, I hope he really enjoys it!