system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do must Irish last names start with Mc?
gpt
It means "Son of" 

So someone with the name McDonald would be the son of Donald.

Mac means the same thing (MacDonald vs. McDonald). Mc is a short version.

People would be named things like Sean MacDonald, because their father was named Donald. It was a way of showing which family you belonged to, by identifying yourself with your father (the head of the household). Eventually, it stopped being literally your father as people kept their last names for longer.