It is a term that refers to an Irishman and originated from a group set up in the 1800s called the "Fenian Brotherhood", who aimed to achieve
Irish independence. Brits use it sometimes as a derogatory term for the
Irish who instead take it as a compliment.
In modern times in Scotland, rangers fans call Celtic fans "fenian" and some Scottish
people believe the term refers to Roman Catholics, but this is incorrect.
In recent years, in n.Ireland (n.irland is
irish land thats still occupied by britian.). Unionists have used the term "fenian" to try to insult
Irish. Due to the vast majority of unionists being protestant and vast majority of Irish being catholic, some unionists now use the term to refer to any catholic, but again this is incorrect use.