In my opinion, it is not worse than 'all' London, in the same way as there are peers of DD paying much, much more in Edinburgh, York, Exeter, Bath etc, but the real challenge as PPs' mention is supply in Dublin. Less and less UK 1st years are securing Trinity Halls accommodation and if you do what we did, which was apply for both TC and private, you get rejected for TH. School boy error on our part, but we also didn't want to take the risk on zero accommodation when R1 offers came out so bit the bullet on Kavanagh C at around £9k for 38 weeks. Blessing really, the location itself was a massive bonus, resulting in her staying their 2 years being back and forth campus in 15 mins to enable her to maximise academic/social side of life via walking and frequent luas lines.
But, students work there. And they work during their summers too, often staying on RoI for their jobs, Laidlaw scholarships etc, so they make the most of their 51 week contracts if using private halls.
Re 4 years, anecdotally from my DD's year, most have secured year abroad for Y3, the offering is vast and open to most courses (mandatory on my DD's), history included; Vienna, for example, is popular for history. ALL students are able to access Erasmus, with partner universities non EU also on the list. Some territories are deemed a cheaper cost of living so self-fund only to offset the higher costs of other regions/spread the funding pot. Middle East as an example. Subject to where a student chooses, this can be a year of savings vs TCD Y3 costs, unless they choose US where student will need to demonstrate circa £20k - £25k savings plus incur the higher costs of US living.
Re Y4, students who have played an active uni life by 'giving back' through their society roles, club participation etc, get priority dibs with campus accommodation. Basic, but much cheaper for final year.
TCD is not for everyone, it's grown up living from the get go but for those who are independently minded, want to be around smart kids due to the high entry requirements alone, who are (in my opinion from what I've seen), pretty driven, active in society life and work/play hard, it's a great place to study if you have one of those!