The Ellén Trechend is a three-headed

monster A monster is a type of imaginary or fictional creature found in literature, folklore, mythology, fiction and religion. They are very often depicted as dangerous and aggressive, with a strange or grotesque appearance that causes Anxiety, terror ...

referred to in

Irish mythology Irish mythology is the body of myths indigenous to the island of Ireland. It was originally Oral tradition, passed down orally in the Prehistoric Ireland, prehistoric era. In the History of Ireland (795–1169), early medieval era, myths were ...

. It is mentioned in the text ''

Cath Maige Mucrama The ''Cath Maige Mucrama'' (in English the Battle of Mag Mucrama) is an early Middle Irish language tale which forms part of the Cycles of the Kings. Content The cast includes several major figures from Irish pseudo-history, Ailill Aulom, his so ...

'' (''The Battle of Mag Mucrima'') as having emerged from the cave of Cruachan (

Rathcroghan Rathcroghan () is a complex of archaeological sites near Tulsk in County Roscommon, Ireland. It is identified as the site of Cruachan, the traditional capital of the Connachta, the prehistoric and early historic rulers of the western territory ...

County Roscommon County Roscommon () is a Counties of Ireland, county in Republic of Ireland, Ireland. It is part of the province of Connacht and the Northern and Western Region. It is the List of Irish counties by area, 11th largest Irish county by area and Li ...

) and laid waste to Ireland until it was killed by the

Ulaid (Old Irish, ) or (Irish language, Modern Irish, ) was a Gaelic Ireland, Gaelic Provinces of Ireland, over-kingdom in north-eastern Ireland during the Middle Ages made up of a confederation of dynastic groups. Alternative names include , which ...

poet and hero Amergin. Its name is difficult to interpret: ''trechend'' means "three-headed", but ''ellén'' is an obscure word. One translator interprets it as a "swarm of three-headed creatures"; Whitley Stokes offered a "monstrous triple-headed bird" (

Old Irish Old Irish, also called Old Gaelic (, Ogham, Ogham script: ᚌᚑᚔᚇᚓᚂᚉ; ; ; or ), is the oldest form of the Goidelic languages, Goidelic/Gaelic language for which there are extensive written texts. It was used from 600 to 900. The ...

''én'', "bird"); while T. F. O'Rahilly identifies it with

Aillén Aillen or Áillen is an incendiary being in Irish mythology. He played the harp or timpán and would lull his victims into a deep sleep with his music. Character Called "the burner", According to the most frequently repeated story, Aillén har ...

, the fire-breathing monster fought by

Fionn mac Cumhaill Fionn mac Cumhaill, often anglicised Finn McCool or MacCool, is a hero in Irish mythology, as well as in later Scottish and Manx folklore. He is the leader of the ''Fianna'' bands of young roving hunter-warriors, as well as being a seer a ...

. In P.W. Joyce's '' A Smaller Social History of Ancient Ireland'' he tells how the Sidhe of Cruachan opened on

Samhain Samhain ( , , , ) or () is a Gaels, Gaelic festival on 1 November marking the end of the harvest season and beginning of winter or the "Celtic calendar#Medieval Irish and Welsh calendars, darker half" of the year.Dáithí Ó hÓgáin, Ó hÓ ...

and a crowd of horrible goblins rushed out, along with a flock of copper-red birds who were led by a monstrous three-headed vulture.
P. W. Joyce, "A Smaller Social History of Ancient Ireland", 1906 It is very possible that the vulture is the Ellen Trechend as the copper-red birds are also mentioned in ''Cath Maige Mucrama''.

References

Aos Sí Cycles of the Kings Fairies Irish folklore Irish legendary creatures Tuatha Dé Danann {{Celt-myth-stub