The native species of wild or crab apple in the Blessed Isles is known by the Latin name malus sylvestris or forest apple. In its wildest and least domesticated form, it still has thorns and, in our reality, the fruit is bitter and only provides good eating when cooked or turned into preserve. While the apple in general appears in the folklore and mythology of the Blessed Isles as a symbol of fruitfulness and as a means to immortality, the instance that should probably be borne in mind most is that of Connla, who was given an apple by his fairy lover that became his sole source of food and drink while he pined for her and her fairy land.
Or, as phrased in official Unnatural Resources guidance:
Reports from Rangers with second sight are explicit about how (bigender) apple tree spirits do their best to entice men to eat their fruits. All Ranger reports are clear about how the malus sylvestris observed in the Otherworld bear thorns that can stick through leather and make it very difficult to retrieve enticed colleagues. Those few who manage to sample the apples, whether Rangers or missing people who have been lucky enough to return, have developed symptoms identified as the Changeling Effect. As for the pining, everyone who has eaten Otherworld apples (and survived) says that they are the most delicious food that has ever been and they would happily kill anyone and everyone if they thought it would bring them another taste, as well as how much they wish they could go back to that particular tree.