"I work in the morning."
Translation:Oibrím ar maidin.
Good question. But it is always "ar maidin". For further "confusion" see below link (initial mutations, right at the beginning):
(Another discussion thread stated, that "ar" in the meaning of state/position does not cause lenition, and that this version of "ar" stems from an obsolete preposition "for".)
No. Even if it were used, sa maidin would be used rather than san maidin, since maidin doesn’t begin with a vowel. Remember that prepositions rarely have complete one-to-one correspondences between languages.
A form of the preposition i is used in Tá sé ina mhaidin (“It is morning”, more literally “It is in its morning”).