Why are Dublin people known as Jackeens?

Dublin people are not commonly referred to as "Jackeens." The term "Jack" or "Jackeen" is an archaic and occasionally derogatory term used to refer to a person from rural Ireland, particularly those from the western province of Connacht. It carries a connotation of unsophistication and lack of urbanity, and its usage has declined significantly in recent decades due to its negative connotations.