The accents in GOT don't really stand up to being matched with regional accents in the UK or Ireland.
For example, Ned & Robb Stark and Jon Snow do sound appropriately northern, with what sound like Yorkshire accents to me. (Bean used his own accent, Madden is Scottish and Harington from London). However, Bran and the girls have more sort of 'received pronunciation' - ie posh - accents, as did Catlyn (the actress is from the north of Ireland). This could possibly be because Catlyn is a Tully and from a different part of Westeros.
Most of the Winterfell staff also sounded northern as did the Karstarks and Umbers. However, despite being supposedly bannermen to the Starks, the Boltons sound posher. Possibly because Michael McElhatton (Roose) is from Ireland and toning down his accent just as Iwan Rheon (Ramsay) is toning down his Welsh accent
Alfie Allen is from London but also adopted a northern accent, which makes sense if he grew up with the Starks. However, while most other Ironborn sounded like him, Patrick Malahide played Balon with an RP accent.
Robert and Stannis Baratheon have quite northern-sounding accents but their brother Renly sounded much more RP to my ears. This is perhaps because the brothers were largely brought up separately and was also perhaps deliberate, to emphasise how he was different from his siblings.
Arguably the wildlings should've had Scottish accents (as the Wall was inspired by Hadrians wall) but they largely sound like the other northerners. Ciaran Hinds is from Belfast but puts on a northern English accent. The actor playing Thormun is Scandinavian; some of the northern Scottish Islands do have a Scandinavian influence.
While Kings Landing is supposedly far south of the north, some of its denizens such as Janos Flynt and Davos Seaworth have northern accents (Liam Cunningham is from Dublin but has adopted a Newcastle accent). Then you have Aidan Gillen, who does a really dodgy Irish accent, despite being Irish and sounding genuinely Irish in all other roles. I can only assume that this is to emphasise Littlefinger's chameleon character and the fact that he doesn't really belong anywhere.
That's before you even consider how people who weren't born on this side of the narrow sea, such as Varys, Daario and Thoros sound largely the same as those who were.
