Congrats on the offers!
I've a UK educated DC at TCD, not History but a joint Hons course. I'm going to be vague as they wouldn't want to be outed.
At the time, we positioned it as do 1st year, but said if by Christmas they didn't like it, could reapply with grades in hand to UCAS before Jan 31st. Has my family in Dublin which has helped. I think the biggest difference is size - UCL is huge, TCD is very small.
Loves it however. Sees themselves staying on after graduation.
What to know?
Finance: yes, overall Uni is far cheaper, but there's no finance. The cost of living, especially food, and accommodation is high (think €1000 a month for rent, maybe plus bills) as long as you can completely self-finance, plus part time jobs are possible and well paid. Public transport is cheap.
Accommodation is a nightmare. My DC was lucky to have a 1st year place in Trinity Hall, as long as they had a 1st Round offer (which they did). They met lots of people there and now live with them . Most 2nd years secure a place in eg The Point by Christmas. Don't underestimate this, and definitely don't rely on a place if you haven't one. Kavanagh had spaces a few days ago. Get on every waiting list. Lots of student halls in D7 and 8. Some places may free up when Round 2 offers are released next week, but these will be single figures. More chance if she will share a room.
Uni set up: 2 semesters. So finish late at Christmas, but have a month off, so go back way after everyone else, then early finish in May, and starts back Sept 9th. They don't therefore align well with UK student friends. Erasmus is an excellent opportunity, although my DC has Irish citizenship, so check it's open to all.
Also the terms has affected their ability to chase UK internships as the dates don't always align, but there are Irish ones too.
Culturally: Dublin is somewhat like, but also very unlike the UK- so be aware that there will be norms they will have to navigate. The Irish are very, very friendly and open. Everything can be bought there of course, contrary to what US parents believe- there's an Social Media group site for mostly US based Trinity parents that is helpful but also includes amusing tips like ' make sure you pack supermarket bags' for them and wild claims that things like stationery cannot be found easily.
Weekends/ social life: There are many societies to join, but outside of them, there isn't much of nightlife/activity beyond the library on campus at all. Just the Pav for drinks, no SU bar. Almost all sports are off site. At weekends, Dublin students live at home and go out with existing local and schoolfriends, non-Dublin go home, and the US ones (and there are many of them- 40% of TCD students are from overseas). The upside is that my DC was invited home by many friends.
Overall it has been fabulous for them. You can hold both places as late as possible. Have you considered hedging by deferring the UCL place or is it too late? I assume you would then declare the TCD 1st year if completed.
Happy to answer any other specifics. UCL is also amazing, what a brilliant dilemma.