In Irish folklore, the tree whose berries and branches were associated with protection against fairies is the rowan.
The European rowan, Sorbus aucuparia, has bright red berries and a long history as a protective or apotropaic tree in Celtic and wider European traditions. Folklore treated it as a safeguard against witches, malevolent spirits and, in some accounts, fairies. Rowan branches could be placed near doors or carried as charms.
The belief is not a botanical claim that the berries repel supernatural beings; it is a traditional folk belief. Rowan was valued partly because its conspicuous berries and hardy growth made it visually distinctive, while its protective reputation spread through stories, charms and local custom.
A common mix-up is with hawthorn, elder and holly, all of which also have important roles in Celtic folklore. Hawthorn is especially associated with fairy places, but rowan is the tree most characteristically linked with protective magic. The Irish literary tradition also connects rowan with the ogham letter luis.