The 24-year-old midfielder has joined the club for the remainder of the season from Aston Villa.
His arrival comes just hours after reports emerged that the deal had fallen through because he had failed a medical.
Ireland becomes Alan Pardew's second January signing after Frenchman Hatem Ben Arfa completed a permanent move despite not playing since suffering a double leg fracture just four games into a season-long loan deal.
Cork-born Ireland, who has maintained his self-imposed exile from international football despite a series of approaches from boss Giovanni Trapattoni, has struggled to rediscover his best form since making an £8million switch to Villa from Manchester City last summer.
He made his debut for the midlands club at St James' Park on August 22, and it was a day he will remember with little pleasure as Newcastle romped to a 6-0 win in which Carroll scored his first Premier League hat-trick.
But Newcastle failed in a late bid to lure Charles N'Zogbia back to St James' Park.
The Magpies made a last-gasp offer for the 24-year-old Frenchman, who left the club in a £6million move to Wigan two years ago after waging a concerted campaign, but saw it swiftly rebuffed by the Latics.
And Spanish striker Xisco has returned to Deportivo La Coruna on loan from Newcastle.
The 24-year-old moved to St James Park for £5.7m in 2008 and has had just 18 minutes of Premier League action this season.
Xisco said on the Depor website: "I'm returning to my home. Deportivo have given me everything and I've come here focused on helping the team reach their objectives."