It’s just bùth that’s translated as a shop.
It’s ann am that’s translated as in. The preposition originally used to be just am (or an), am bùth could mean both the shop as well as in a shop, but it seems that was too ambiguous so the preposition got repeated: ann am bùth, lit. there, in a shop or in it, in a shop to disambiguate.
Before the definite article ann an, ann am always changes to anns so in the shop would be anns a’ bhùth (where anns means in and a’ bhùth is the shop in dative case).
See also Ann an on the Akerbeltz wiki.