The 31-year old from Edinburgh, who has been capped three times for Scotland, pleaded guilty at the city's Justice of the Peace Court today (May 15) to striking Damian Holahan on the head in Malones Irish Bar in Forrest Road on February 3 last year.
His defence solicitor, Kathleen More, told Justice Julia Toch, that her client was 'a man of great talent on the football field'. She added that he was out of contract at the moment, but could afford to pay a financial penalty.
Riordan's solicitor said he had been enjoying a night out with his brother and had consumed some alcohol when he and Mr Holahan had accidentally banged into each other on the crowded dance floor.
She said: "His recollection of the events is a bit vague.
"Mr Riordan misjudged the situation and struck the complainer on the face. He regrets it very much indeed."
Justice Toch fined Riordan £430, reduced from £480 because of the guilty plea. He has until July 10 to pay the full amount.
Riordan signed for HIbs in 2001 and went to Celtic in 2006. He returned to the Easter Road side in 2008 and left in 2011 before brief periods with a Chinese side, St Johnstone, Bristol Rovers and Alloa Athletic.