Interesting that I'm not the only one to ask. Back in 2008 a lad asked the same question and a guy named David answered, "
I think any of these could be appropriate in different circumstances - now, I'm not working from any source as reliable as a native speaker, but I'd use imímis in the sense of "let's go (away)", téimis as "let's go (to someplace/something)" and ar aghaidh linn as something like "let's get going", i.e. a call to action of sorts which may or may not involve actually going to/from somewhere."
I remember the words "Téimis" and "Imímis" when I studied Irish time ago (gotta get back to it!). But according to David, "Ar aghaidh linn" is a call to action, as in "Let's get going!", as well as CaoimhínSF wrote. (This is actually what I'm looking for.
BTW, just curious, do regions difference these expressions, as in, let's say, Connemara, since I'm interested in the Connacht dialect.
grma arís
