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 featured regularly for the club's under-18 team last season and was introduced as a 16-year-old second-half substitute in the 3-1 defeat at Watford on the final day of last season.

He impressed at Vicarage Road with some powerful headers and calmness when the ball came his way, including great awareness to position himself on the line and clear what would have been a certain goal otherwise.

The 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.

O'Brien, whose older brother James is on the books at Birmingham City, spent the previous years of his footballing life at Cherry Orchard FC.

He has represented the Republic of Ireland at all levels up to and including U17, and is one of several Irish youngsters currently making their way in Derby's Academy.

In October 2009 Mark 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.