Iriekpen began his career at Premiership side West Ham, but moved to Swansea City in August 2003.
He has been a first-team regular at the Welsh side, playing in 137 senior games and scoring eight goals.
United are also showing an interest in several young players - including Newcastle pair Phil Cave and Andy Carroll.
Cave is a 20-year-old centre-back, with Carroll a promising youngster who may be heading to Glanford Park on a season-long loan.
Blackburn midfielder Joe Garner, who ended last season on loan at Carlisle, is also reported to be a target.