I've been well amused for a while by the seemingly mad passive aggression apparent in "'S e do/ur bheatha" ("You're welcome", but literally "It's your life", right?) and "Mar sin leat/leibh" ("Goodbye", but literally "like that with you"/"back at you!"). I understand the second is from having dropped the customary "Beannachd leat/leibh" (~"Blessings upon you") that used to precede it, but would be curious to know what's up with "You're welcome".
Anybody else got any favourite odd literal translations?
Also, while I'm writing, if it's dropping the quasi-religious goodbyes (beannachd leibh), did Scottish Gaelic ever have the same religious hellos as Irish Gaelic, i.e. "God be with you", "God and Mary be with you", etc.?
Well, the first one, although literally today sounds like it’s your life (or he is your life), most probably comes from a completely different Old Irish phrase – rot·bia de bethu, lit. let (a lot) of life be with you (word-for-word to-you may be (plenty) of life) and what was meant was something like may you live long.
Already in Old Irish times the phrase went through a drastic reduction and changed to día do bethu which was reanalyzed as something like god is your life but was kept being used when welcoming someone and then evolved to modern is e do bheatha, both in Irish and Scottish (although its usage differs in the two languages).
You can read more about the history of this phrase in the Jesus is life? article on the Akerbeltz wiki, which nicely summarizes it:
So paraphrasing the idiom, it's really just a very Gaelic way of saying live long and prosper.
As for religious greetings (like the Irish Dia (’s Muire, is Pádraig, is Bríd…) dhuit), look at the Feasgar math or how to start an argument Akerbeltz article.
TLDR is: yes, probably such greetings were used in Scotland, but if so, they went out of use in the past long gone. Since in 18th–19th century Gaelic would be mostly used by people who knew each other (and knew they both speak Gaelic), they’d most probably just greet each other by name (A Dhòmhnaill, fada on uairsin – Donald, long since then; or A Mhàiri, dè do chor an-diugh? – Mary, what’s up today?) and default to English or Scots when meeting a stranger, so most of the modern greetings are newer inventions anyway, as the old ways of greeting a stranger in Gaelic went out of use with little traces of them left today.