Born in Dublin, the 25-year-old started his career with Blackburn Rovers and enjoyed loan stints with Stockport and Sheffield United before heading to Preston North End.
It was during his time with the Lilywhites that he earned his first of six international caps for the senior Republic of Ireland team.
With 58 appearances for Preston, he made the short trip to Burnley in 2011 where he went on to feature 75 times for the Clarets – 31 of which coming last season when he helped Sean Dyche’s team reach the top flight.
Speaking to barnsleyfc.co.uk, Treacy said, “I'm delighted. The clock was ticking down to the start of the season. I was wanting to get back onto the pitch and back to playing football again and Barnsley have given me the opportunity to do that and I am delighted to be here.
"When I was talking to Danny Wilson he laid out where he wanted to take the Club and I was happy to come and be a part of that. Hopefully we will be able to get out of League One as soon as possible.
"I'm an old school winger. I like to get the ball and run at people and get crosses into the box.
"I watched the game against Sheffield Wednesday on Saturday and I was impressed with the lads. I'm sure we will all be doing what we can to get Barnsley back into the Championship."
