"You intend to listen to the stories."
Translation:Tá fút éisteacht leis na scéalta.
lena means "with his" (or "her" or "their") - or, in this exercise tá fút éisteacht lena scéalta means "You intend to listen to his stories".
le + a (possessive pronoun) becomes lena
le + an (definite article) becomes leis an
le + na (definite article) becomes leis na
leis is also the 3rd person singular masculine prepositional pronoun ("with him"/"with it").
If you want to use "an earlier exercise" as the basis of your question, please quote the actual exercise that you are referring to.
Irish doesn't have infinitives. Ag is not used to translate an English infinitive. Ag is used with bí and the verbal noun to translate the present progressive. It is also possible to translate this construction using a gerund rather than an infinitive - "you intend listening to the stories" (though it might not be considered idiomatic in all versions of English).