The Scot has also signed Liverpool defender Andre Wisdom on a one-year loan deal.
The deal to retain Foster will be considered significant since he won both the players' and supporters' player-of-the-year awards at the end of the last campaign, while Wisdom was added as a consequence of a recommendation from Brendan Rodgers, the Liverpool manager.
"I've spoken to Brendan, whom I've known for a very long time and whom I trust implicitly," said Irvine. "He said Andre would do a good job for us. That was reassuring. You can see the player, but you want to find out what they are like as a person. Brendan spoke highly about him."