Given the literal translations elsewhere should this not be I'll see you 'again'
It's a bit of a package this one, such a commonly used idiom that the literal 'again' isn't thought of. After all, in order to see someone at a later point in time, you will indeed see them again.
The translation you give is "a-rithist" - that is again not later
"nas anmoch" is later - surely?
Otherwise perhaps re-phrase the question as
"See you again, Ruairidh"
This should surely be chi mi fhathast thu, Ruairidh or See you again for a-rithist?