I think it's a fair enough observation @magicgirl , though like many experiences at education institutions, so much is dependent on the cohort they're entering, how prepared the DC is to put themselves out there and how wide their prospective friendships extend, beyond the course contact circle.
Anecdotally, my DD has fortunately not had this issue (from London), perhaps due to getting lucky with her small but very international course that includes home Irish students and her wide range of interests that she's able to continue. She's also lived with a mix, that included Irish students who went home every weekend, but this also meant that she grew her Irish network by being included in their 'home related' plans, like 21st's, visiting, short tourist breaks etc. Likewise, our home has felt like a youth hostel over the last 3 years (not a bad thing), reciprocating the hospitality she has enjoyed.
She's been able to store her stuff locally whilst being on her year abroad this year, a semester or full year that most of her student friends take resulting in returning year 4 being much easier as they're all in the same boat with added external experiences and perspectives. TCD encourage this where possible, subject to academic performance, though YA was mandatory for her.
If anything, from her observations, the cliquiest group has been the Brits (aptly named the Brit Group) that has included some US students. They have had a very different outlook, spending budget and have not been as inclusive as one would think, a group she has steered clear of.
Like with most things, it is what you make of it, but she wouldn't recommend for those DC in need a little more handholding, or prefer the collegiate nature of some other unis. It's grown up living from the get go, but in her case, a very enriching one, so far.