The former Manchester City player was again left out of Lambert's matchday 18 for Saturday's Barclays Premier League clash at Arsenal.
Ireland's last action was in the Capital One Cup semi-final with Bradford nearly a month ago. But Lambert reiterated that his decision is based on footballing reasons.
He said: "I have to pick as squad that I think will benefit the football club. He (Ireland) has featured up until recently.
"It is my decision. Stephen has got a contract, I've had chats with him and we get on well."
Lambert has a trio of injury concerns - including skipper Ron Vlaar - to contend with ahead of next Monday's home encounter with champions Manchester City.
Defender Vlaar was ruled out of the game at the Emirates Stadium with a calf injury along with club-record signing Darren Bent and experienced goalkeeper Shay Given.
Lambert said: "Ron got a knock on his calf, Darren got a kick on his foot and Shay has a little strain."
- Sports & Recreation