Brian Boru was an 11th century Irish king on Earth. As "High King of Ireland," he commanded a thousand Irish warriors against the Vikings at the Battle of Clontarf. He was the founder of the O'Brien dynasty.
Miles O'Brien was a direct descendant of this king, whom he would play in the holosuite. (DS9: "Bar Association")
The authors of the Star Trek Chronology (page 10) note that: "Ironically, victorious Irish king Brian Boru is killed by retreating Vikings in the aftermath of the battle".