Known as the Celtic Warrior, Collins is the most successful male Irish boxer in recent professional boxing history, having held the WBO middleweight and super-middleweight titles simultaneously and never losing a fight as champion.

After reaching 16–0, Collins stepped in as a substitute in a WBA middleweight title fight after Michael Watson was injured in training, and fought 12 rounds against Mike McCallum in Boston in 1990.

In 1992, Collins lost a majority decision to Reggie Johnson in a closely contested slugfest for the vacant WBA middleweight title (which had been stripped from McCallum because he signed to fight IBF champion James Toney).

Collins then moved to Belfast under the management of Barney Eastwood before basing himself in England where he joined Barry Hearn's Matchroom Boxing.

Alongside him was Paul "Silky" Jones, his sparring partner and good friend who later went on to become WBO light-middleweight title holder.

In May 1994, Collins finally won a world title by defeating Chris Pyatt by stoppage in five rounds to become the WBO middleweight champion.

Collins defeated the then unbeaten long-reigning champion Chris Eubank in Millstreet, County Cork, Ireland, in March 1995, by unanimous decision (115–111, 116–114, 114–113), to win the WBO super-middleweight title.

In their September 1995 rematch in Cork, Collins performed brilliantly, changing his usual fighting style by adopting wild, brawling tactics throughout which Eubank really struggled to deal with.

In the summer of 1997, Collins reportedly stated in the press that he had no motivation left, as he had spent the best part of his career chasing Roy Jones Jr. for a fight that had been promised to him many times.

In 2014, Collins joined the Army Reserves, and in 2017, transferred to 253 Provost Company[4] of the 4th Regiment Royal Military Police in London where he had been living for the previous 20 years.