O'Brien was educated in Marian College, Ballsbridge. His brother Kevin is also a member of the Ireland team. Their father Brendan played 52 times for Ireland. His sister Ciara O'Brien is in the Ireland women's hockey team.
Niall first gained recognition playing domestic cricket in Leinster for the Railway Union Cricket Club earning a call up to the Ireland A team. From there he established himself as Ireland's number one wicketkeeper. O'Brien himself has said that he has a fiery temperament. On 17 June 2004, he angered Brian Lara when Lara refused to walk after being apparently caught behind. Ireland caused an upset when they went on to win the match, with O'Brien scoring 58* off 57 balls. In 2006 he was banned for one ICC Intercontinental Cup match after an outburst criticising the groundsman at an Intercontinental Cup game with Scotland because of condition of the pitch.
O'Brien has spent many an Australian summer playing for Mosman Cricket Club in the Sydney Grade Cricket competition. O'Brien also assumed a part-time coaching role while at the club and considers the time spent in Australia as some of the most vital parts of his career. He also played two seasons of first grade for North Sydney in the Sydney Grade Competition in 2003/04 and 2005/06. He also played a season with the Mosman club as an 18-year-old.
Along with fellow Ireland player Ed Joyce, O'Brien made a series of impressive performances for Ireland before starting to take part in English cricket with Kent. O'Brien found multiple chances to perform for his county as fellow wicket-keeper Geraint Jones has found himself performing more often for the England cricket team.
After being released by Kent he signed a one-year deal with Northamptonshire for 2007. In the same year, he joined the Essel Group's Indian Cricket League (ICL) to represent the Delhi Jets team for one season. O'Brien became the first player to play international cricket after playing in the ICL when he played an Intercontinental Cup match against United Arab Emirates in March 2008; he went on to score 174 in Ireland's nine-wicket win.
O'Brien's batting statistics with Northamptonshire
At the end of the 2008 season, O'Brien was named Northamptonshire's player of the year and was rewarded for his consistent performances with a three-year contract with the club. A representative of Northamptonshire CCC said "Niall, who has just returned from international duty, was in sensational form last season with the bat, scoring over 850 championship runs and piling on the runs in one-day competitions in a new pinch-hitter role". In a tour match against Australia during the 2009 Ashes in July 2009, O'Brien made scores of 30 and 58 in a match Northamptonshire lost. During his half century, he scored seven boundaries off the struggling Mitchell Johnson.
O'Brien set a World Twenty20 record with his 4 dismissals against Sri Lanka in the Super 8 stage of the 2009 ICC World Twenty20. O'Brien was selected in Ireland's 15-man squad for the 2011 World Cup to be hosted by Bangladesh, India, and Sri Lanka between February and April. During the tournament he became the third player to score 1,000 ODI runs for Ireland after William Porterfield and his brother Kevin. He passed this landmark in Ireland's five-wicket defeat to India. In the same match, O'Brien and Porterfield established a new records for Ireland's third wicket in ODI, putting on 113 runs together. Ireland were due to play two ODIs against Pakistan in May, however O'Brien suffered an injury to his hand and was forced to miss the fixtures.
At the end of the 2012 English County season, O'Brien left Northamptonshire after six seasons. In October 2012, it was announced that Leicestershire had signed O'Brien in a three-year deal.
On 16 February 2015, he smashed unbeaten 79 runs off 60 balls with 11 fours, guided Ireland to the victory against another Test match playing nations, the West Indies. This was Ireland's third consecutive victory over test playing nations in three consecutive world cups. O'Brien's score was helped to beat mighty West Indian pace attack and finally Irish won their first match of 2015 ICC Cricket World Cup.