I saw this article on the BBC website today about the use of Gaelic in Caithness.

http://www.bbc.co.uk/news/uk-scotlan...lands-17515935

At the bottom I noticed;

"In 1707 a presbytery reported to the General Assembly Committee for the Highland Libraries that "there are seven parishes in Caithness, where the Irish (Gaelic) language is used, viz. Thurso, Halkirg, Rhae, Lathrone, Ffar, Week, Duirness."

So they must have been quite a bit of immigration from Ireland to Caithness.
This could possibly explain in the similarities in the accents, especially since I noticed it most in Thurso.