Miles Edward O'Brien, played by Colm Meaney, is a main character in Star Trek: Deep Space Nine. Before DS9, he appeared as a recurring character in Star Trek: The Next Generation. O'Brien is the only major Star Trek character known to be both ethnically Irish and born in Ireland.
According to Colm Meaney, at first O'Brien "was just there, not really established as a character, and that went on for a bit. Appearing on and off in more TNG episodes, it wasn't until the fourth season episode "Family" that Meaney's character was finally given a name. However, Meaney came to like the arrangement of being hired on an episode-by-episode basis, and was hesitant to sign on as a regular on DS9.
The TNG episode "The Wounded" establishes that O'Brien served as tactical officer aboard the USS Rutledge and that he was emotionally scarred by the Cardassians' massacre of hundreds of civilians on Setlik III.
In the DS9 episode "Bar Association", O'Brien jokingly claims to be a direct descendant of real-life Irish High King Brian Boru (according to Irish tradition, all people with the surname "O'Brien" are descendants thereof). Later, he speaks more seriously of fictional ancestor Sean Aloysius O'Brien, who participated in the Coal Strike of 1902 in Pennsylvania, and was shot, then dumped into the Allegheny River. In the episode "Rules of Engagement" we learn that during O'Brien's 22 years in Starfleet he had fought in 235 separate battles and had been decorated by Starfleet on 15 occasions and was considered to be an expert in starship combat.
O'Brien's first appearance in Star Trek: The Next Generation is as the battle bridge flight controller in The Next Generation premiere episode "Encounter at Farpoint" In almost all of his subsequent TNG appearances, however, he is a transporter operator.
O'Brien marries Keiko Ishikawa aboard the USS Enterprise-D in the TNG episode "Data's Day". They have a daughter, Molly, who is delivered by Worf in "Disaster".
The character of Miles O'Brien was transplanted from TNG to DS9 at the beginning of the latter show. In the story, Miles O'Brien transfers from the Enterprise-D to Deep Space Nine, in the DS9 premiere episode "Emissary", to serve as the station's chief of operations. He simultaneously works as chief engineer aboard the USS Defiant, which is assigned to Deep Space Nine in the episode "The Search".
Although Bashir initially irritates O'Brien, the two characters eventually become best friends. In particular, they frequently play darts and fight historical battles in the holosuites.
A few years after Miles is transferred to DS9, he has a son, Kirayoshi, delivered by surrogate Kira Nerys in episode "The Begotten".
At the end of Deep Space Nine, O'Brien and his family depart the station to move back to Earth, where Miles is to serve as an engineering professor at Starfleet Academy. The soundtrack accompanying these moments on screen is to the tune of "The Minstrel Boy."
At various points in TNG, he wears black silver-rimmed insignia (TNG: "Encounter at Farpoint"), black gold-rimmed insignia (DS9: "Emissary"), or lieutenant insignia ("Redemption, Part II"). Sergey Rozhenko calls him a Chief Petty Officer in "Family" (TNG). Eventually, O'Brien receives a distinct senior chief petty officer's insignia and his rank is emphatically identified in "Hippocratic Oath" (DS9).