The Irish midfielder has become the latest departure from Oakwell, following Sam Winnall and James Bree, in the January transfer window.
It’s an issue which boss Paul Heckingbottom believes is the Reds being a “victim of our own success”, as their rise from League One has attracted clubs with bigger budgets.
Hourihane - who was also a target for Sheffield Wednesday - has signed a three-and-a-half year deal at Villa Park, for an undisclosed fee plus sell-ons.
The 25-year-old has been a huge favourite at Oakwell, since joining from Plymouth in 2014, and netted in the 3-2 derby win over Leeds United in his final appearance in a Tykes shirt.
“The memories that I will take from Barnsley will stay with me forever - lifting two trophies at Wembley to name just a few of the special moments that we have shared,” he said.
“It was fitting that in my last game for the club I scored in a superb win over local rivals Leeds United.
“I certainly enjoyed that one and it feels like I am leaving on an incredible high.
“After the journey that we’ve been on and the success that we’ve had, particularly over the past 12 months, it was a very tough decision to leave Barnsley and I had to think long and hard about what was right for my long term career.
“Barnsley has been great to me over my time here and I’d like to think that I’ve been good to you and I leave with the club in a better place. Make no mistake, Barnsley is a club that is on the rise and Oakwell has a very bright future.
“Being captain at the club was the biggest honour that I’ve had in my career so far, and I will be eternally thankful to the club for everything that it has done for me. Barnsley Football Club will always have a special place in my heart.”
A club statement said it wasn’t an easy decision to sell Hourihane, but accepted Villa’s offer as the player could leave for noting in the summer when his contract expired.
“Conor made it clear to the board that he saw the move as an opportunity to further his career and he wanted to pursue the move,” said the statement.
“Under these circumstances, and considering that Conor’s contract was coming to an end in the Summer where he could transfer for free, the Club respected that decision and reluctantly allowed him to move on for a significant fee and sell-on.
“Conor was a hugely popular figure at the Club and we fully appreciate that this news will be met with disappointment from our tremendous supporters.
“The Club firmly shares this sentiment. It is always difficult losing a player of Conor’s footballing and personal qualities, and allowing a player of his calibre to leave the Club was never an easy decision.”