Reports emerged in November that Miller had pancreatic cancer and was receiving treatment in the US, where he played for Wilmington Hammerheads. Tributes for the Cork-born player have poured in on social media.
Celtic tweeted: "Everyone at #CelticFC is deeply saddened to hear of the passing of former midfielder Liam Miller. Our thoughts and prayers are with his family and friends at this time.
RIP Liam, YNWA." Former Ireland teammate Steven Reid said: "RIP Liam Miller.
Was lucky enough to room with Liam on a few international trips.. top player and a great person.
Thoughts and prayers with his loved ones." Former Manchester United defender Danny Higginbotham said:
"RIP Liam Miller. Devastating news. Great player but more importantly fantastic person. So sad. Thoughts with his family."