Donn is said to dwell in Tech Duinn (the "house of Donn" or "house of the dark one"). A 9th-century poem says that Donn's dying wish was that all his descendants would gather at Tech Duinn after death: "To me, to my house, you shall all come after your deaths". The 10th-century tale Airne Fíngein ("Fíngen's Vigil") says that Tech Duinn is where the souls of all the dead gather. In their translation of Acallam na Senórach, Ann Dooley and Harry Roe commented that "to go to the House of Donn in Irish tradition means to die". This suggests that the pagan Gaels saw Donn as their ancestor and believed they would go to his abode when they died. Tech Duinn may have been thought of as a place where the souls of the dead gathered before travelling to their final destination in the otherworld, or before being reincarnated. According to Julius Caesar, the Gauls also claimed descent from a god of the underworld whom he likened to Dīs Pater.
The Christian writers who put together the Lebor Gabála Érenn made Donn (also called Éber Donn) one of the Milesians, the mythical ancestors of the Gaels. The Milesians invade Ireland and take it from the Tuatha Dé Danann. During their invasion, Donn slights Ériu, one of the eponymous goddesses of Ireland, and he drowns in a shipwreck off the southwest coast. Donn is then buried on a rocky island which becomes known as Tech Duinn. In the literature, Tech Duinn is said to lie at or beyond the western edge of Ireland. Tech Duinn is commonly identified with Bull Rock, an islet off the western tip of the Beara Peninsula. Bull Rock resembles a dolmen or portal tomb as it has a natural tunnel through it, allowing the sea to pass under it as if through a portal. In Ireland there was a belief that the souls of the dead departed westwards over the sea with the setting sun.
In the tale Togail Bruidne Dá Derga ("The Destruction of Dá Derga's Hostel"), king Conaire Mór meets his death in Bruiden Dá Derga (the "great hall or hostel of the red god"). On his way to the hostel, Conaire meets three red men riding red horses from the otherworld. They foretell his doom and tell him "we ride the horses of Donn […] although we are alive, we are dead". It has been suggested that Dá Derga and Dá Derga's Hostel is another name for Donn and his abode.
In the tale Tochmarc Treblainne ("The Wooing of Treblann"), the otherworld woman Treblann elopes with the mortal man Fráech, who sends her to safety in Tech Duinn while he embarks on a quest. In this tale, Donn is said to be the son or foster-son of the Dagda. Dáithí Ó hÓgáin notes similarities between the two and suggests that Donn was originally an epithet of the Dagda.
Folklore about Donn survived into the early modern era. In County Limerick, a Donn Fírinne was said to dwell in the hill of Cnoc Fírinne (possibly meaning "hill of truth") and was associated with the weather. Thunder and lightning meant that Donn Fírinne was riding his horse through the sky, and if clouds were over the hill it meant that he was gathering them together to make rain. In County Clare there was a Donn na Duimhche or Donn Dumhach ("Donn of the dunes"), who "was also often encountered as a night-horseman".
Hy-Brasil - famed island to the west of Ireland that features on many maps but was never found.
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