At Old Trafford in the 1990/91 season, Irwin played at right back but was soon moved to left back the following season with the purchase of Paul Parker. Irwin fitted into the position and that is were he remained for the rest of his career. He soon made the role his own at International level, making his debut for Ireland in 1990 against Morocco and playing in the 1994 World Cup in the USA.

Irwin was a skilful attacking full-back, deloying swerving crosses and clever overlapping runs. He also had one of the most powerful shots in the game and scored some great goals in his time - In 1993 a 25 yarder in the 1-0 win at Coventry, 1994 - his mazy dribble and finish against Wimbledon in the FA Cup 5th round and in 1993 his free-kick against Liverpool at Anfield. In fact, up until the arrival of David Beckham he was United's first choice free-kick taker.

Denis always gave a very solid performance, so much so that he rarely ever had a bad game. Importantly he was always cool-headed on the big occasions. As the 1990's progressed success came to Old Trafford and Irwin quietly went about his business whilst collecting the medals. Even in the 1998-99 season, the onset of advancing years was never noticeable and it was business as usual for Irwin who was consistent and reliable as ever. At 33 Denis was as good, if not better than he'd ever been.

United's inability to find a consistent penalty taker led to him having to take responsibility for 4 of United's 5 penalties that season. His only blemish was his outrageous sending off by Referee Elleray at Anfield, which cost him a place at Wembley FA Cup Final with Newcastle because of suspension. That aside, it was another flawless season topped off with him appearing in the Champions League Final win over Bayern Munich to complete the famous 1999 treble.