The word fenian means many things, depending on who you ask. In early Irish mythology, the Fenians (or Fianna) were a band of warriors protecting the ancient Irish people. Later, during the Irish War of Independence, the label was used by groups of revolutionaries fighting to free Ireland from British control.
In the war’s aftermath, the word took on a darker meaning in the north of Ireland, which remained part of the United Kingdom. It became a slur — a derogatory term for Irish Catholics.
I've never heard tis term before - need to look more into the history.
I thought Highsnobiety had a good definition:
The word fenian means many things, depending on who you ask. In early Irish mythology, the Fenians (or Fianna) were a band of warriors protecting the ancient Irish people. Later, during the Irish War of Independence, the label was used by groups of revolutionaries fighting to free Ireland from British control.
In the war’s aftermath, the word took on a darker meaning in the north of Ireland, which remained part of the United Kingdom. It became a slur — a derogatory term for Irish Catholics.