We saw the movie in 2008 and just saw the musical at the Orpheum. We loved them both. Terriffic music. I really liked the non-storybook ending. However, if we continue to play out the characters' lives, I think the storybook ending might come to pass. Here is my reasoning...
The relationship between Guy and his NYC expat girlfriend is clearly too tempestuous to last. I think he goes to NYC, gets back together with her, but they only last 18 months, 2 years tops.
As far as Girl's relationship goes--any husband who can't get along with such an all-around delight as her is bound to be worthless. Maybe he just can't adjust to life in a new country, I don't know. But no way are they going to stay reconciled.
So, within 2 years, Guy and Girl are both free to take up with each other. Whether that happens is not a sure thing. If he finds stardom, all bets are off. Likewise, if he stays in NYC, it's probably going nowhere. But there is a significant probability he is headed back to Dublin in 2 years, tail between he legs. And if that happens, the next step is obvious.