Long spent the second-half of last season on the sidelines, suffering an anterior cruciate ligament injury on January 1, on his Premier League debut against Newcastle United.
Prior to that, the 25-year-old had a series of loan spells at the likes of Portsmouth, Rochdale and Accrington Stanley, having come through the Cork City youth structure.
Barnsley manager Lee Johnson was keen to stress that Long was fully fit after a long spell out.
He said: “He’s had pretty much a full pre-season, and I think he’s played seven or eight Under-21 games. He’s ready now.
“There’s nothing wrong, stability wise, with the knee. He’s fit and ready to go. He’s hungry, he’s come in and he’s ready to play first team football again.”
Johnson also believes that Long could make a significant difference to a defence that has struggled for solidity throughout the campaign, conceding 29 goals in their 17 league games.
He added: “He’s a centre-half, he’s going to be playing centre-half for sure. He’s in there to compete. We’ve conceded too many goals, so he’s come in to shore up that back-line.
“He’s a really good player, solid, strong, composed on the ball, and he’ll be a good addition to our squad.”