The 33-year-old Irishman penned a one-year contract at Loftus Road on Monday after being released by Aston Villa earlier this summer.
"This is a great signing for us," Redknapp told the club's official website. "He's a top-class defender and a great guy. He's a good character and just what we need.
"Obviously he had his problems last season with injury but there's an agreement in place that if he breaks down, the contract can be ended.
"He's confident he'll be okay and from our point of view, if he doesn't have any problems with it then we've just signed a very, very good centre-half."
QPR fly out to Austria on Tuesday to continue their pre-season preparations.