Chief Petty Officer Miles Edward O'Brien, played by Colm Meaney, is a character in Star Trek: The Next Generation and Star Trek: Deep Space Nine. O'Brien was originally the transporter chief of the USS Enterprise-D. He was later promoted to Chief of Operations of Deep Space Nine. O'Brien is the only major Star Trek character described as both ethnically Irish and born on Earth in Ireland.

The backstory of the character states that Miles O'Brien was born in Killarney, a town in County Kerry in Ireland, Earth, in September 2328.[3] He claims descent from Brian Boru, the famous Ard Rí, or High King of Ireland (and founder of the O'Brien Clan). His father, Michael O'Brien, wanted him to play the cello, so he pursued this and was eventually accepted into the Aldebaran Music Academy (revealed in DS9 episode "Shadowplay"). However, a few days before he was scheduled to start classes there, he enlisted in Starfleet. O'Brien can also be seen playing the cello as part of Data's string quartet early in the TNG episode "The Ensigns of Command". In the DS9 episode "Invasive Procedures", it is revealed he has two brothers.

The TNG episode "The Wounded" establishes that O'Brien served as tactical officer aboard the USS Rutledge during the Cardassian War and that he was emotionally scarred by the Cardassians' massacre of hundreds of civilians on Setlik III. O'Brien does not remember how many Cardassians he killed, because he killed so many (Star Trek DS9: "The Inquisition"). In that episode, it is clear that the classic Irish tune "The Minstrel Boy" plays a major part of his journey as a character: an innocent man thrown into the destructive nature of war. He sings the song in this episode, and much later, in the final episode of DS9 "What You Leave Behind". "The Minstrel Boy" is the first musical theme to be heard in the flashback sequence.

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, a major player in one of the first United States workers' unions, 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", it is revealed 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.