The preposition + article causes eclipsis almost all of the time. In this sentence we have "leis an," so the noun following "an" needs to be eclipsed.
I wonder if somebody could explain why we say "Rithim le do mhadra" and "Rithim leis an mbrat"? Is there a difference between "le" and "leis" or they can be used interchangeably?
We say leis an instead of le an because it is easier to say leis an. (In the case of le na, saying leis na avoids any confusion with lena)
le becomes leis before a definite article - it's not the same leis as the 3rd person singular propositional pronoun leis. And it's not just alternative for le - you can't say leis do mhadra.