"I just don't understand how they can know so many people. It feels like they each know about a thousand people"
That's the thing, that actually do. There's only about €5million people in Ireland. Everyone knows everyone, or at least knows someone who knows the person they are looking for it's tiny.
Honestly (and don't want to sound creepy), if I wanted to find an English lady, in Ireland about 3 years, working in retail in, with a female manager and in only about 3 possible towns in the country, it wouldn't be difficult to find you by Friday.
It really is that small. I grew up in the country, everyone knows everyone. I now live in Dublin which is more anonymous but having been here several years, it's also getting small.
I suspect you are in a town where the majority of your co-workers are from the area, they have never left, they went to school together, their kids now go to school together, they've been in the same job for life since they were young and there isn't much going on other than people so they talk about people. With a retail wage and the cost of living in Ireland, there isn't much money left to do much outside Ireland. They are mostly happy people. It's their way of life.
It's not malicious gossip, it's limited options and experiences. Sense of community is very strong in Ireland, there's no doubt about that but I can't imagine anyone is deliberately trying to exclude you. Most of those people are probably actually your manager's friends, and then it's awkward because someone who doesn't know then is there listening to the conversation. I know it shouldn't be but that's the way it is.
You might be happier if you got a job where confidentiality is a priority - i.e. medical receptionist, insurance company, local bank etc. The staff there won't be as free to discuss people's personal lives as much.
If there's any multinational companies in the area too, they've gave a more diverse workforce so the local element will be more muted.
I don't think it's necessarily confined to Ireland, if you were to take an entry level, non skilled job (I don't mean that to be derisory, it's just factual) in any area of the world where all the staff were born and raised in the area you would have the same issue, albeit Irish people will probably talk more.
It's not a problem they have with you, it's just their way of life. Social connections are heavily prioritised.
All that aside, this job is making you miserable. The reasons is really irrelevant- you're unhappy so you need to do something different and now you know the area to avoid - schools, creches, the post office, undertakers etc all going to give you a similar experience - try places more suited to what you value.
There's nothing wrong with you.
There's nothing wrong with your co-workers.
It's just incompatibility.
They are happy. You are not. So if you want change, you'll have to be the one to make it.