Ferguson, who has represented Northern Ireland at senior international level, but only in a friendly, has decided to stick with the nation of his birth.
The Derry-born youngster, who played for the Under-21s in their 2-0 defeat by Serbia on Tuesday, told Newcastle's official website, nufc.co.uk: "A lot of stuff was said in the past with people thinking I was going to switch to the Republic of Ireland and in the summer, I had a couple of options.
"I just wanted to take my time and think what I wanted to do, and a lot of people jumped the gun when I wasn't in one of the squads.
"But my heart is with Northern Ireland and always has been. I've represented them since I was a kid and I want to play for them for as long as I can.
"I played for the Under-21s the other day and I was happy to do that.
"It definitely wasn't a decision I wanted to rush into, but a lot of people probably didn't see it that way."