"I am living on a hill."
Translation:Tá mé i mo chónaí ar chnoc.
Because of the preposition ar.
ar is a bit unusual, in that it can cause lenition, eclipsis or no initial mutation at all. The entry for ar in the FGB says:
(In references of a general nature it does not normally affect initial letter of following noun, e.g. ar muir, ar cíos, ar cosa in airde, "on sea", "rented", "galloping". In qualified or particularized references it lenites, e.g. ar mhuir na beatha, ar chíos mór, "on the sea of life", "at a high rent". Eclipses in a few instances, e.g. ar gcúl, "backwards")
This is a "particularized reference", so chnoc is lenited.
While I see the logic of a "particularized reference" here as a reason for lenition ... what's the difference to "Ritheann an bhean ar an bhféar." (=on the grass, looks quite particularized to me too ...) then, where eclipsis is caused. Or the difference to "Tá an bia ar an mbord" (to take one of the very early examples) ... Any ideas? The discussion about the farm situated to the left didn't help me with this issue either: https://forum.duolingo.com/comment/17631890