However only 16% at TCD get a first whilst this figure is 42% at Durham
I would see this as a good thing? Lots of info as @turkeyboots has said on the other thread. The level of students' academic ability will be comparable to your DS's grades in hand, and for those able and willing, will be putting themselves for the Foundation Scholarship papers in Y2, Laidlaw research awards and more, lots of extra stuff for History UGs to apply for. And if the preference is not to apply, then you get access to a decent study programme/contact hours where work is marked, feedback is productive and being a humanities student does not make you feel like a 2nd class citizen (DD's words, not mine).
Not fussed about QS rankings ourselves, more how successful are students in getting first rung on ladders post UG studies (or onto PG if that's their objective).
My own anecdotal experience with a DD in Y3 of 4 with a wide peer group across many subjects is that they've been pretty successful so far, including internships within the EU, US as well as Ireland/UK, lots of interesting opportunities within law and Civil Service as well as the standard corporate graduate programmes.
So much of the early application process is anonymised now, but our personal experience is that the TCD name carries strong weight still internationally and a student does benefit from wide access to Erasmus opportunities if they want it, that can offset the nightmare that is accommodation. I have zero costs during Y3 of DD's year abroad with the exception of 2K EUR for tuition fees.
It's a pain having to wait for the CAO R1 offers so late (was 8th Sept for us post covid with a 16th Sept start), but if he's able to hold his nerve and you know where you stand re accommodation etc, then he can withdraw Durham right up to matriculation with no penalties, or turn down his CAO offer and not incur TC/KC halls fees.
Good luck!