The defender has signed for the Blades for a fee in excess of £4million
John Egan has joined Chris Wilder's side from Brentford after two years with the London club.
The 25-year-old Irishman, who played one match on loan for the Blades in 2012, has made appearances in the Championship, League One and League Two and has been capped twice for his country.
After starting out his youth career in Ireland, Egan joined Sunderland in 2011 and was loaned to Crystal Palace, Sheffield United, Bradford City and Southend United while on the Black Cats' books.
In 2014, the 6ft central defender joined Gillingham after interest from a range of Championship and League One clubs. Egan hit the ground running in his debut season, scoring five goals in 52 appearances and then in 2016 was named in the League One PFA team of the year, sealing a move to Championship side Brentford.
With six goals in 63 appearances for the Bees, Egan also inherited the captain's armband from Harlee Dean at the start of last season, when he guided the team to 15 clean sheets and 9th place in the Championship.
Right-footed Egan made his debut for the Republic of Ireland in 2017 against Iceland and has played at both centre-back and right-back during his career.