Though no longer used as a deliberate slur, this term is sometimes perceived as insulting to or by the Irish. It originated in the Royal Navy in the 1800s, during the time of sailing ships.
Slang definitions & phrases for Irish pennant
Irish pennant
noun phrase
The end of a rope, sheet, etc, carelessly left loose or trailing off a boat or ship: Always loose ends. You know what they call them in the Navy? Irish pennants
[1883+ Nautical; the form Irish pendant is found by 1840]