I don't have a specific accent I see them having (everyone sounds English when I read in my head, lol), but from their description perhaps something Asian (Indian, Pakistani, etc). Or maybe Northern African, European (French, Spanish, Greek, etc), Arabic. It could be anything, really! But if you're already multi-lingual and can do other accents, then there's a good chance you could learn to do one you don't do at the moment.
haha that would be perfect actually, considering I'm half English and a quarter Pakistani. Arabic and Indian accents come easily what with my having been surrounded by many family members who speak the same way. Do you picture the Dornish as an Arab-esque people? Tyrion describes the various ethnic traits of the Dornish at some point if I recall correctly..