The reason the Scots accents in Outlander strike me so oddly is because I've been around a fellow who is a true Highlander for many years and am used to his accent and way of speaking. He's a Gaelic speaker from one of the islands (don't recall which offhand) and his pure vowels and lilting voice couldn't be more different from Lowland speech. Yes Highland accents are often perceived as sounding "Irish" because they come from the same process: Gaelic speakers learning English.

Thing I was thinking about, regarding Outlander, is that the boundary between Scots and Gaelic would have been much further south in 1743 than today. Would that have meant Gaelic speakers learning their English from Scots speakers, rather than from Standard English speakers as it has been in the more recent past?

(Sorry I'm using "English" and "Standard English" in the ways understood in the wider English-speaking world, not meaning the specific dialects now spoken in England.)