http://www.independent.ie/sport/soccer/fifas-eligibility-rules-explained-29647415.html
Q. Who can a player play for when he has never played international football?
A. A player can play for any country for whom they hold a permanent nationality.
Ireland's a really poor example, even FIFA statues can't resolve that easily.
The answer in the QA is simplistic - at a high-level, it's true, but as we have seen there are provisos to that in Sections 6 and 7 which are applicable to players who don't hold a straightforward permanent nationality. Otherwise, if the answer was all there was, Sections 6 and 7 would be redundant and not part of the statutes.
And actually, after just reading the article, it says absolutely everything I've been saying - it even states that Januzaj may have to wait until he's 23 to be eligible!