Brynjolf is a Nord living in Riften, yet he has a unique accent that is not found anywhere else in Skyrim, nor is it spoken by any other Nord. We know nothing of Brynjolf's background or upbringing other than he is a thief and has progressed to a high rank in the Theives Guild.

The accent is comparable to a Scots accent of real life Earth. Scots are a branch of ancient Brit (branch of Celt) which in Tamriel is best represented by the Bretons. Could it be that Brynjolf was raised in High Rock and that his accent is a Breton one? We know that at various points in Tamriel history, Nord families have settled into parts of High Rock and stayed there. We do not however know as to what extent they kept their Nord family names & genetic identity. We have an example of Nords living in High Rock through the surname Early-beard, but info again is very vague.

I am unconvinced that Brynjolf's accent is a regional Skyrim accent as it is a one off. We have another Breton in Skyrim who is the personal cook for Jarl in Markarth, but it would seem that is representing the social elite of Breton nobility rather than the more tribal rooted country folk. We also see Reachmen with Celtic sounding names which could support a Scottish influence. I am quite excited at the idea of the native tongue of the Bretons sounding Scottish as it opens up the opportunity for voice actors such as Sean Connery and Ewan McGregor.

I am also suspicious if the island of Roscrea is Tamriels equivalent of Ireland. The name Roscrea is also coincidentally the name of a town located in the very centre of Ireland, Roscrea is located in the Sea of Ghosts which runs the northern shoreline of Tamriel, It was once annexed by Solitude, suggesting a close proximity to Solitude which is located close to the High Rock border. We know Beth' loosely base the footprint of Nirn on Earthly influences prior to making it into an alien world, so if this is the case, it could also mean we could have some Irish sounding voice actors if the island is ever brought closer into Tamriel lore.

Thoughts?

You're probably over-thinking the accent - Skyrim is in general inconsistent with accents, and the decisions seem to be on a character/voice actor basis than the voice director asking for any strict adherence.

I'd imagine High Rock in general is an amalgamation of English, Irish, Welsh, Celtic, Scottish, Gaelic, etc. cultures, customs and accents - it fits in with their representations and how High Rock is described. The decision they made back in Morrowind to use largely French names for Bretons confuses me - I think it was part of their effort to unTolkienify TES at the time, but now the explanation seems to be western High Rock culture like Anton Virane's home of Daggerfall uses French names.