The 26-year-old arrived at Bramall Lane from Middlesbrough in January 2010, and scored five goals along the way, but has now agreed to be part of new Hornets manager Sean Dyche's plans.
His best time at the Blades arguably came under the leadership of Gary Speed. The midfielder had a decent start to the 2010/2011 season, producing his best performances at Hull and Leicester.
The Irishman didn't have the best of relationships with former manager Micky Adams, playing very few games under him.
His form dropped considerably when Adams was in charge, despite scoring in the new boss's first game at Burnley. His inconsistency began to frustrate Blades fans.
Danny Wilson commented: "Some of the lads are ambitious and want to play in the Championship, as you would expect, and we have had a few nibbles. What I must also stress is we need to bring players in too, this isn't just a one-way process."
Image copyright to Action Images