From TYI, Rules for Eclipsis (in relevant part):
“The initial consonant of a noun is eclipsed in the dative singular following a preposition with the article.” One of the examples they give is:
leis an mbata “with the stick”
Should I infer from this that the same applies to prepositional pronouns with the article? Or is it just the multi-functional “leis” being used instead of “le”?
No, if there’s “leis” meaning “with him” and coincidentally a following article, there is no eclipsis:
Dúirt mé leis an focal ceart a scríobh = I told him to write the correct word.
Only if “leis an” means “with the” there’s eclipsis:
Dúirt mé leis an bhfear an focal ceart a scriobh = I told the man to write the correct word.