I understand how ἐπ᾽ εἰρήνης πρὶν ἐλθεῖν υἷας Ἀχαιῶν works. However I’m a bit confused by τὸ πρὶν.
It seems to mean “previously.” Does it work like οι παλαι (men of old) - “the thing [neuter] of yesterday [πριν]” i.e. “previously”?
Yes it just means “formerly.” It doesn’t work exactly like οι παλαι, where oi serves to make οι παλαι a masc.nom.plural noun, for here το adds nothing to the adverb itself, except to mark it as an adverb. το is sometimes used like this with adverbs in Attic as well as in Homer.
The second πριν in this verse is used as a conjunction governing the acc.&inf. construction.
To have the two of them intensifies the difference between how things were in time of peace (ἐπ᾽εἰρήνης) and now—typically Homeric pathos, and very effective don’t you think?