The 28-year-old Hull City defender qualifies for both countries through his grandmother and initially opted to play for the Republic but was only capped in friendlies, meaning he was not tied to the country.
Former Northern Ireland manager Nigel Worthington first approached the defender in 2011 and he was called up by new boss Michael O'Neill for the last two World Cup qualifiers but was forced to pull out through injury.
As he prepares for his first involvement with Northern Ireland in Wednesday's friendly against Malta, Bruce, son of Hull boss Steve, said: "I'm not naive enough to say I'm not aware of it but for me what's in the past is in the past and I'm just looking forward to the future.
"I've declared myself for Northern Ireland, Michael's given me a fantastic opportunity to play international football. Being a footballer's a short career so I'm going to make the most of the opportunity and hopefully do as well as I can.
"My granny's from Bangor so I always knew, when I hadn't played a competitive game for the Republic that I was still available, and when that phone call came I had a long think about it.
"Unfortunately for Nigel he lost his job but Michael's come in and taken the bull by the horns and cracked on with the talks we've had. I made a decision and I'm just looking forward to playing for Northern Ireland.
"I've had pressure all my life. Obviously the situation at Hull, where my dad's the manager, has been a bit of a pressure cooker as well so I've had a few things like that to deal with over the years and pressure's something I don't really think too much about.
"It's water off a duck's back. I think any footballer will tell you that when you walk over that white line, what people do on the terraces, you're quite oblivious to it."
Wednesday's match will be O'Neill's eighth in charge, and the former Shamrock Rovers boss is still looking for his first win after three defeats and four draws.
St Johnstone's Alan Mannus will start in goal, while Huddersfield midfielder Oliver Norwood was forced to withdraw with a bruised foot.