O'Brien and Bashir in their costumes for the program
The Battle of Clontarf was a military engagement fought at Clontarf in Ireland in the 11th century. In this battle, High King Brian Boru led one thousand Irish warriors against the invading Viking horde.
Miles O'Brien, who claimed to be a direct descendant of Boru, owned a holoprogram of the battle and enjoyed reliving it in Deep Space 9's holosuites, taking the role of Brian Boru. According to O'Brien, this battle was like the Battle of Britain, "only with swords". (DS9: "Bar Association", "It's Only a Paper Moon")
The Star Trek Encyclopedia (4th ed., vol. 1, p. 73) noted that this battle took place in 1014.