My three started out with very 'naice' English accents thanks to where we lived in the south of England. Then we moved to Ireland, when they were aged between six and two.
Within two weeks of moving to DH's native mid-West they'd lost their slightly posh English accents altogether and picked up a version of the lovely, soft local accent. I'm sure my northern English vowels sounds helped a bit 
I am not a fan of any Dublin accents (sorry!) but I love the accent my lot have now. To Irish people they sound Irish with a bit English in there somewhere, to English people they sound Irish.
We have a video tape of the three of them chatting away when we first moved here, their accents are completely different now. It's amazing what they do to fit in.