Maybe he chose it as a variant of McClare, which is a Scotitish surname (perhaps the fact that the one episode took place in Scotland had something to do with it), though I admit looking it up I found more using McClare than MacClare.
Maybe he should've chosen a name of Welsh origin (examples: Prewett, Owens, Morgan) since Flintshire is a Welsh name and they got a Scottish courtesy title (and presumably Duneagle) through Shrimpie's grandmother. Otherwise it would make more sense if it had been a Welsh grandmother who gave them the title of Flintshire and the MacClare family being always based in Scotland with the title of Newtonmore.
It was mentioned in the Series 3 Christmas special that the title is Welsh, but they live in Scotland because Shrimpie's grandmother was the Countess of Newtonmore (Scottish) in her own right and was the owner of Duneagle Castle... which is why the family are there.
What do you think?