Mark Leo O'Brien (born 20 November 1992) is an Irish former professional footballer who played as a defender. He played in the English Football League for Derby County, Luton Town and Newport County and in the Scottish Premiership for Motherwell. Club career Derby County O'Brien started his career at Cherry Orchard, before he was spotted by Derby County scouts in the Dublin & District Schoolboy League. After signing for Derby, O'Brien became a regular in the club's under-18 team in 2008–09, before making his professional debut as a 64th-minute substitute for Lewin Nyatanga in a 3–1 defeat away to Watford on 3 May 2009. He was earlier named Derby County Academy Player of the Year at a club awards ceremony. He also won the 2009 Republic of Ireland Under-16 Player of the Year award. In November 2009, it was announced by Derby County that O'Brien had successfully undergone heart surgery to correct a valve problem, and that he would be able to resume his career following a three to four-month recovery period. The problem had been discovered during a routine medical scan for academy players. The operation was carried out at Leicester's Glenfield Hospital. Following a successful recovery, he was an unused substitute in Derby's final first-team game of 2009–10 as they ran out 2–0 winners against Cardiff City at Pride Park. In 2010–11, O'Brien returned to being a regular in the club's under-18 team and reserve team, and was also part of The Central League Central Division title winning team. O'Brien appeared twice for the first-team, both as late substitutes in league games against Barnsley and Ipswich Town in October and November 2010 respectively. On 6 August 2011, O'Brien made an appearance in the first game of 2011–12 season in a 2–1 win at home to Birmingham City, replacing the injured Russell Anderson in the 35th minute. O'Brien continued to start in subsequent league games against Watford, Blackpool and Doncaster Rovers, as Derby won their opening four league games for the first time since 1905. However, this run was ended against Burnley, as an O'Brien defensive error contributed to a 2–1 defeat. On 13 September, O'Brien signed a new four-year contract, extending his stay at Pride Park until the summer of 2015. He made 21 appearances for the club until he was ruled out for the season in December, after damaging his cruciate ligaments in training. O'Brien won the