Ellén Trechend

According to the medieval tale Cath Maige Mucrama (The Battle of Mag Mucrama), the Ellén Trechend was a monstrous three-headed creature that emerged from the dark cave of Cruachan — known today as Oweynagat, the 'Cave of the Cats' near Rathcroghan in County Roscommon. Once free, it rampaged across Ireland, devastating the land until it reached Mag Mucrama, where it was finally killed by the Ulster poet and warrior Amergin.

The name itself has long puzzled scholars. 'Trechend' clearly means 'three-headed', but 'ellén' resists easy translation: one reading takes it as a 'swarm' of three-headed beings, the 19th-century Celticist Whitley Stokes proposed a monstrous triple-headed bird, and T. F. O'Rahilly linked it to Aillén, the fire-breathing creature that terrorised Tara before being defeated by Fionn mac Cumhaill.

Oweynagat looms large in early Irish literature as one of the chief 'gates to the otherworld', a cave from which supernatural hosts and monstrous beings were said to pour forth, especially at Samhain. The Ellén Trechend's emergence from this cave situates it within that same mythological complex — a distinct creature with its own tale, anchored to the cave mouth that already appears on the map as the Oweynagat entry.