John & Mary's is a shop owned by married couple John and Mary O'Leary, who not-so-secretly dislike each other.
The shop is seen when Ted and Dougal are protesting at the local cinema and the former asks the latter to buy a pair of handcuffs as well as Ted wanting a box of firelighters in the aftermath of Mr. Benson's whistle being stolen.
Among the many incidents involving the couple, Mary attempts to kill her husband by dunking his head in a bucket of water, which he passes off as her washing his hair, and he locks her in a giant wardrobe. They also come to blows over a shotgun which goes off when Ted leaves the shop.
The shop is seen for the final time when Ted and Dougal go in and find that Mary screaming at John after she has been locked in a wardrobe with him making the excuse that it's for her own protection from the Beast of Craggy Island.