The 24-year-old left-back, who spent most of last season on loan at Doncaster, has agreed a two-year contract at Fratton Park.
Pompey boss Paul Cook told the club website: “We’re delighted to bring Enda to the club. He brings a lot of quality and adds balance to the left-hand side.
“It’s important now that he kick-starts his career and has aspirations to progress and perhaps get into the Ireland squad.
“He’s got all the talent in the world, but he has to knuckle down with us and be the player that I believe he can be.”