Enda Stevens says joining the Blades was an 'easy decision'.
Chris Wilder's third signing of the summer jumped at the chance of moving to South Yorkshire, despite winning the League Two title and Players Player of the Year at Portsmouth last season.
And Stevens says the chance to work under Wilder for a second time in his career was a big lure.
The 26-year-old, who was officially confirmed as a Blade on Monday, remarked: "I'm pleased to sign so early into the summer.
"I spoke to the manager after the season had finished and following negotiations I came to Sheffield, had my medical and immediately signed on the dotted line.
"The move has been in the pipeline for a while, but my mind was made up once the Blades came calling. I know a lot about the club and more about the manager so it was an easy decision for me personally.
"United were keen straight away and it was a move that really appealed to me. I played under the manager at Northampton and it was an enjoyable month I had there on-loan from Aston Villa."
The Dublin born left-back, who has signed a three-year deal, added: "I like the way his team plays, I've experienced playing against his teams and it is always a tough game, they work hard and are always in your face.
"I enjoyed my time in the region at Doncaster, so I'm looking forward to returning to South Yorkshire and I'm well informed of what to expect at Bramall Lane."