The former Manchester City player was again left out of Lambert's matchday 18 for Saturday's 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."







