@turkeyboots has covered much information @Becauseurworthit , your DS running both UCAS and CAO applications parallel at least gives him wider choice, if he is able to hold his nerve to the end.
Mine has just graduated from TCD (from London), not a Maths but humanities student, so do feel free to ask any questions on experience if needed. The good thing is that A levels have recently been realigned better to CAO points, but as Turkeyboots has said, the heavy subscribed courses so have cut offs/lottery. Conversely, places can become available in subsequent offer rounds.
Accommodation is tough. Mine was in private for years 1 and 2, abroad for Y3 (so zero cost, Erasmus funded) and £6k campus accommodation cost for Y4. This tends to be prioritised for students who have participated well in student life - societies, scholars etc. Her campus flat was shared with 4th/5th year medics, engineers, maths students.
Student life was strong, lots of European/US in her cohort. And lots of Irish family homes to visit at weekends. Travel home and overseas very easy, it worked well for us.
Cost of living is high. No concessions for students in bars, tougher rules for under 21s etc.
Finally re different timings of offers/exam results, DD relinquished her UK place post Round 1 (around Sept 10th) but she had secured accommodation at both UK and Dublin, no penalties for letting go of either.
Forfeiting the access to Student Finance was a big deal, but net net she has ended up with a significantly lower overall cost of study, but only possible due to our parental support.
Good luck to him!