O'Brien began his football career with Ballyfermot club Cherry Orchard.[1] In the summer of 2006 he moved to England to take up a scholarship with Birmingham City. After only one year with the club, during which he was part of the youth team which reached the quarter-final of the FA Youth Cup, he signed a two-year professional contract.[3] He played regularly for Birmingham's reserve team and was an unused substitute as they won the Birmingham Senior Cup in 2008.[4] O'Brien joined Conference North side Solihull Moors on a month's loan in January 2009,[5] but the loan was interrupted when the player sustained a pelvic injury a couple of weeks later.[6]
After his contract with Birmingham expired, O'Brien spent pre-season with League Two club Bradford City. He impressed during trials, scoring on his debut in a pre-season friendly against Burnley, and on 4 August 2009 signed a three-month contract.[2] He made his professional debut for Bradford on 12 August as a second-half substitute in a 3–0 loss against Nottingham Forest in the League Cup.[7][8] His debut in the Football League came three days later as a late substitute in the goalless draw at home to Port Vale.[9] O'Brien scored his first goal for Bradford on 22 August, in a 5–4 victory away at Cheltenham Town.[10][11] At the start of 2010, O'Brien had surgery on his nose to alleviate breathing issues, and therefore was restricted to substitute appearances.[12]
O'Brien signed a new 18-month contract with Bradford City on 29 October,[13] but was released on 26 July 2010, 10 months early.[14] He subsequently returned to his native Dublin where he signed for his boyhood club St Patrick's Athletic.[15]
He signed for Longford Town in July 2011, and was released in July 2012.[16]
While a Cherry Orchard player, O'Brien captained Ireland's under-16 side. According to his manager Vincent Butler, he was "excellent for us this season and in most of our matches was the best player on the pitch",[22] and his performances with that team earned him the eircom/FAI Under-16 International Player of the Year award.[23] He graduated to the under-17s,[1] missing out on the elite qualifying round of the 2007 UEFA Under-17 championships through injury,[24] under-18s,[25] and, in the 2008–09 season, to the under-19s.[26]
