The 27-year-old right-back had his contract at the League One club cancelled by mutual consent.
Barry Cotter, pictured for Barnsley. (Image: PA)
Cotter made 66 appearances for the Tykes, having joined them from Shamrock Rovers in January 2023.
He spent the first half of this season on loan at Notts County, but has not played for the League Two outfit since September.
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The Irishman now returns to his native homeland to link up with Tiernan Lynch’s side, who have picked up four points in their opening three games of the new League of Ireland campaign.
On signing for his new club, Cotter said: “The standard of the league has improved again since I last played and when I saw the squad that the gaffer has assembled, it looked very strong.
“It’s something that really appealed to me and I know I can help the team to challenge for trophies this season.
“I’m very much looking forward to getting started.”
Barry Cotter in action during his time at Ipswich Town. (Image: Pagepix)
Cotter started his career in England with the Blues, joining Town from Limerick in January 2018.
However, he made just four appearances during his spell at Portman Road, impressing against Barnsley in his one Championship start.
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There were boos when Cotter was taken off in the 56th minute of a 1-1 home draw with the Tykes, with McCarthy subsequently announcing he was walking away from the club during a desk-thumping post-match press conference.