Erin go bragh
March 17th is the feast day of the patron saint of Ireland, St. Patrick. In the United States, it's also the day of green beer (and green everything else). And it's also the day we see the most people turning to their dictionary to look up Erin go bragh.
Erin go bragh means "Ireland forever," but the original Irish phrase was "Erin go brách" (or "go bráth") which translates literally as "Ireland till doomsday."
It's an expression of loyalty and devotion that appears to have been first Anglicized during the late 18th century Irish rebellion against the British:
"A private letter states, that the Insurgents, on gaining the first advantage, hoisted the standard of Rebellion, with the motto Erin go Bragh, which, however, was soon demolished by the successful reinforcement that arrived to the aid of the Yeoman Cavalry." — Whitehall Evening Post (London, England) 26-29 May 1798