Joseph Martin O'Brien (born 17 February 1986) is an Irish professional football coach and former player who played as a full-back and defensive midfielder. He is currently head coach of League of Ireland Premier Division club Shelbourne.

O'Brien played for Bolton Wanderers, Sheffield Wednesday, West Ham United and Shamrock Rovers, and was capped five times by the Republic of Ireland at senior level between 2006 and 2012.

In 2021 he became the assistant manager of Shelbourne and remained in the role until June 2025, when he became interim head coach of the club following the resignation of Damien Duff. O'Brien got the job on a permanent basis in July 2025.

O'Brien started his career with Bolton Wanderers in 2004 after starting out his first year scholarships in 2002.

To gain further experience, O'Brien was loaned out to League One team Sheffield Wednesday for the most part of the 2004–05 season, making 15 appearances and scoring two goals, including one on his debut. After extending his loan spell at the club twice times, O'Brien was named as captain for the last game of his loan spell in honour of his contribution.

Upon returning to Bolton Wanderers from a loan spell at Sheffield Wednesday, O'Brien made his Premier League debut on 15 May 2005 against Everton when coming on as a late substitute in place of Fernando Hierro. He was given his first start in Bolton's colours in their away UEFA Cup 1st Round tie with PFC Lokomotiv Plovdiv in September 2005. In the 2005–06 season, O'Brien was called up by the first team following a defensive crisis of injuries and that moment on, he became a first team regular, making thirty-three appearances in all competition. Initially playing as a centre-midfielder, O'Brien changed to a right-back role and made an impression, leading to Manager Sam Allardyce described him as a "dominant force at right back in the Premiership".

O'Brien missed most of the 2006–07 season due to a knee injury. By October, O'Brien was expected to return from a pitch in a months time, with a help from an American doctor Dr Frank Jarrell, a pioneer in the field of spinal reflex analysis. However, his recovery was delayed for another 3–4 months, causing him to miss the rest of the season. Having regained his fitness, it wasn't until on 23 September 2007 when he made his return for Bolton, making his first start and playing the whole game, in a 1-0 win over Tottenham Hotspur. Since making his return from injury, he played most of the 2007–08 season in his natural midfield role as opposed to the right back position which he had been in two years previous. Despite further injury concerns, O'Brien went on to make 26 appearances in all competition.

In July 2008 he was given the number 8 shirt previously worn by Iván Campo and started well when he provided two assists against Stoke City and Arsenal. However, injuries restricted his appearances throughout the 2008–09 season. O'Brien finished the season, making eight appearances in all competitions.