system
A chat on subreddit /r/me_irl
human
Of course everyone doesn't know each other. The nuance you are missing is the parochial nature of small Irish societies. Particularly rurally. Most people live and work in or around the same areas they grew up in. So, even somewhere like County Dublin, the most densely populated area in Ireland by some distance, people tend to stick around the villages/neighbourhoods where they grew up. People start businesses there, drink in the same pubs, support the same GAA teams, and so on. Because of this, quasi tribal nature of Irish society, there is a high chance of knowing someone (perhaps not intimately) if they're from in or around the same area as you. Big cities do not have the same parochial attitudes - many people in big cities are 'blow ins', they're not from there, their roots are not there, they don't have generations of families who lived in the area going back hundreds of years (if not more). In Ireland that's exactly what we have. Chicago was founded in 1837, Guinness was founded in 1759 - we have a lot more years of culture and ancestry to draw on here.
gpt
It's really interesting to see how hard a concept this is to grasp for people not from Ireland.

I'm from the north, in Belfast. Still everyone I end up talking to knows someone I'm either friends with or one of their friends. Every single person I've actively talked to in the past 5 years has been at most 2 degrees of separation away. Of course not everyone knows everyone. But when things are as tight knit as they are, you just kind of happen to speak to people who know each other. Still blows my mind sometimes. My last job had me working with the dad of someone I went to school with, entirely by coincidence just because we live in the same general area.