Sunderland
Reddy, as a striker, signed for Sunderland from Kilkenny City in 1999. He spent five years on Wearside; between 2001 and his departure in 2004, he was loaned out to Swindon, Hull, Barnsley, York, and twice to Sheffield Wednesday. He made 18 appearances for Sunderland, scoring twice, and he scored a late equalizer against Middlesbrough in the league.[1] His other Sunderland goal came in the League Cup against Luton Town.[2] Manager Peter Reid declared Reddy to be the "most exciting young player" he had ever worked with. According to Swindon boss Andy King, Reddy had been valued at £5 million in 2001.[3] He received the club's Young Player of the Year award and was called into the full Irish squad in the same season. Michael's career at Sunderland ended after he ruptured his medial ligament in a training ground accident.
Grimsby Town
In July 2004, Reddy signed for Grimsby Town as one of the first new signings under newly installed manager Russell Slade. He was a huge signing for the cash-strapped and relegation-hit club, as was the addition of winger Andy Parkinson, who had joined the club a few days previously. During Reddy's first season, he was still recovering from a medial ligament injury. Reddy was used as Town's striker, mainly playing alongside Martin Gritton, Matt Harrold or Colin Cramb throughout this season. He finished the season by scoring a hat-trick against the Kidderminster Harriers. He managed nine goals throughout the season, and the club finished in 18th place.
The 2005–06 campaign began with Grimsby. His was with the new signing of Gary Jones. Reddy and Jones managed to find the net 31 times throughout the season. During the season, the club defeated both Derby County and Tottenham Hotspur in the League Cup, before losing to Newcastle United. Michael was involved in all three games. Reddy was named League 2 Team of the Year.
Reddy's season was interrupted by a hip injury in March, and as a result, he did not complete another ninety minutes for the remainder of the campaign. The Mariners fell out of the automatic promotion ranks on the penultimate day of the season. Reddy was deemed fit enough to take part in the play-off campaign. Gary Jones and Ben Futcher's goals defeated Lincoln City in the semi-final stages. Slade opted to play Reddy from the start in the final at the Millennium Stadium against Cheltenham Town, but he was substituted due to receiving a concussion after a clash of heads. Grimsby attempted to use Junior Mendes and Gary Cohen to help cater for Jones in attack, and the club lost the game 1–0.
After failing to secure promotion, his manager, Russell Slade, resigned, and Reddy requested to be put on the transfer list. This was followed by several press reports linking him with a move to the newly promoted Premiership side Sheffield United. It was Leeds United and Bristol City who made more of a forward approach for the player. Grimsby had also made the signing of striker Isaiah Rankin in the hope of combining the pair in an attempt to make a second assault on promotion from League Two, but Michael still had a persistent hip injury. Reddy managed to play the odd game, and soon Slade's replacement as manager, Graham Rodger, was dismissed by the club after a 'poor' start to the season. In turn, the club appointed Alan Buckley as his successor. The new Grimsby boss stated that he wanted to keep Reddy, who had by now undergone hip surgery. With Reddy interested in leaving Blundell Park, he subsequently ignored contract talks in favor of potential offers from Bristol City. Leyton Orient had made an unsuccessful approach for Reddy's services.