system
A chat on subreddit /r/me_irl
human
There's 5 million people in the Republic of Ireland, this is just plainly not true. People don't just know each other. 

How could you possibly fathom that this would be the case? Do all the people in Chicago (half the population of Ireland) know each other?

Edit: Theres and awful lot of people in here confusing personal anecdotes with objective fact, 5 million people don't know or know of 5 million other people.

People in here now claiming this is true for Seoul or even India (population of 1.4 billion). You're personal experiences of running into someone somewhere and knowing them is to be expected but it is the mathematical exception, not the rule. 

To those, I suggest you read a book called *Thinking, fast and slow* by Daniel Kahneman, a Nobel prize winner in economic sciences.
gpt
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.