The City striker qualifies through his grandparents and holds an Irish passport, and although he played for England at U20 level in 1997, he has not represented England in a full international.
Macken, 26, who struck City's second in their 3-1 friendly win over Lazio on Saturday, was therefore cleared by FIFA to wear the green of Ireland.
He told the City website: "It's an amazing honour for me and my family and I look forward to justifying my selection if given a chance in the game.
"I've always had a desire to play for the country of my family background and nationality but until now have never been given the opportunity."
The £5m signing from Preston added: "I’m looking forward to joining up with what is a fantastic squad and over the next two years will be doing my utmost to establish my place in the team for the World Cup qualifiers."
Irish boss Brian Kerr revealed he had been watching Macken for some time after becoming aware of his eligibility.
But Kerr chose to leave Macken's team-mate Richard Dunne out of the squad as he opted for a number of prospects yet to make a competitive appearance for the Irish.
However, City youngsters Paddy McCarthy and Willo Flood are in the U21 squad for their friendly on Tuesday.