Relegation-threatened Livorno have sacked Domenico Di Carlo after just 88 days in charge and re-appointed Davide Nicola.
Virgil van Dijk claims he knows nothing of interest from English clubs and is keen to prove himself in the Champions League with Celtic.
Ireland, 27, follows Austrian international forward Marko Arnautovic to the Britannia Stadium after he completed his move from Werder Bremen.
Stoke boss Mark Hughes managed Ireland at Manchester City and the club's chief executive Tony Scholes hopes the midfielder can recapture the form he showed then.
Scholes said: "Stephen is a player that Mark knows really well and when the opportunity arose he was keen to add him to his midfield and attacking options.
"Arguably the most successful period of Stephen's career to date was when he played for Mark at Manchester City and we hope that he can rediscover that form."