"The cat runs around the restaurant."
Translation:Ritheann an cat timpeall na bialainne.
So, I've got to drill it into my head: 1. compound prepositions like around make one inflect the object in question - or has "timpeall na mbialann" actually become the object in this case. 2. Inflecting the noun in this case requires the genitive and that makes one have to inflect 'an' or 'na' to the right form.