Dublin-born Mark only became a full-time Academy scholar at the start of 2009/10 season - but made his first-team debut before then.

Centre-back O'Brien had featured regularly for the club's under-18 team and was introduced as a 16-year-old second-half substitute in the 3-1 defeat at Watford on the final day of the 2008/09 season.

He impressed at Vicarage Road with some powerful headers and calmness when the ball came his way.

His debut came just days after O'Brien had been named as the club's Academy Player of the Year for 2008/09.

And that was yet another accolade in the youngster's fledgling career as he was also crowned the Republic of Ireland U16 International Player of the Year earlier in 2009.

In October 2009 Mark suffered a setback when he underwent surgery to repair a valve in his heart, keeping him out of action until early 2010.

A month later he was rewarded with a professional contract on his 17th birthday, committing his future to Derby County on a three-and-a-half year deal.

He resumed full training by the end of the season and was an unused substitute for the first team on the final day of the season against Cardiff City.