Preposition choices are often driven by regional/dialectal variation, personal preference, or just spur of the moment whim. I usually say "at" a restaurant but I do sometimes say "in". The meaning is clear. I say "pouring rain" but the English for some reason add a redundant preposition and say "pouring with rain" which I always think sounds like it could be pouring with custard or beer on another occasion. It's called colour and it's great.
I would usually only say "IN the restaurant" if it was juxtaposed in context with some kind of outside option, e.g. "Shall we go to the food trucks, or would you rather eat in a restaurant". Even then I'd probably use "at".
Another scenario in which I might say "IN the restaurant" would be if I were talking on the phone with someone who was looking for me, and I was telling them I was "in" as opposed to "out".
Just my American perspective.
A cultural question here: In smaller towns and villages in Italy "the square" is the place to be; it's where everyone hangs out, both young and old, individuals and groups. It's usually also the old market place, even if nowadays there's no longer a regular market and people come to cafés and bars. Now I was wondering if it used to be/ still is like that in Ireland. I've seen the square in Kildare, which kinda seems to fit the picture, but there's too much car traffic and the weather was too cold (in February) for people to sit outside. I haven't seen such a square in Galway (and Dublin, as a city, is of course different). But what about smaller towns and villages?
Not really! :-/ It was quite a culture shock for me when I first visited European cities and saw those large, raised, empty concrete squares! I understand that markets were/are held daily or weekly on them, but in general, we don't have open-air markets in towns in Ireland. And when the first big shopping centre built in Tallaght (in Dublin) was called "the Square/ an Chearnóg", it had no meaning for me! The "squares" in Dublin (Merrion, Mountjoy etc) are in fact green parks with railings around them because they were private, only for the residents of the houses built around that green square. In Dublin, there ARE stalls/stands selling fruit and veg, but that's the length of the street on either side (eg Thomas Street). Having said that, in Kenmare town in Kerry, they have a "village square" because that's where the animal market was (and still is!) held, with animals being weighed there before being sold. So obviously, a larger open space was needed. You can check it out online by doing a search for the Tourist Office/Heritage centre and the Lace Design Museum. You'll see though that it's mainly used as a car park because it's on the same level as the road. So it's not a pedestrian area that could be used by anyone and everyone and the open-air stalls aren't even on it, but a little further away. I hope that I didn't ramble on too much and answered your question! And of course, if anyone else has more information, it would be great to read it. :-)