The Curragh is also home to the Curragh Camp, the chief base of the Irish Army. It's been an army camp since well before Irish independence and was the site of the Curragh mutiny, when officers in the British army based in Ireland threatened to resign their commissions rather than enforce the Home Rule Bill then before the British parliament. This is an almost-forgotten episode in British military history - the closest thing in modern British history to a military coup. The mutiny was a huge boost to Edward Carson's Ulster Volunteer Force and a spur to the formation of the Irish Volunteers and James Connoly's Irish Citizen Army.

My reading of the song has always been that it refers to the army camp rather than anything to do with racehorses - or sheep.