Déisi

Possible presence in Britain

The Déisi Muman are the subjects of one of the most famous medieval Irish epic tales, The Expulsion of the Déisi.^[12] This literary work, first written sometime in the eighth century, is a pseudo-historical foundation legend for the medieval Kingdom of Déisi Muman, which seeks to hide the historical reality that the kingdom's origins lay among the indigenous tributary peoples of Munster. To this end it attributes to "the Déisi" an entirely fictive royal ancestry at Tara.^[1] The term "Déisi" is used anachronistically in The Expulsion of the Déisi, since its chronologically confused narrative concerns "events" that long predate the historical development of déisi communities into distinct tribal polities or the creation of the kingdom of Déisi Muman.^[3] The epic tells the story of a sept called the Dal Fiachach Suighe, who are expelled from Tara by their kinsman, Cormac mac Airt, and forced to wander homeless. After a southward migration and many battles, part of the sept eventually settled in Munster.

At some point during this migration from Tara to Munster, one branch of the sept, led by Eochaid Allmuir mac Art Corb, sails across the sea to Britain where, it is said, his descendants later ruled in Demed, the former territory of the Demetae (modern Dyfed). The Expulsion of the Déisi is the only direct source for this "event". The historicity of this particular passage of the epic apparently receives partial "confirmation" from a pedigree preserved in the late tenth-century Harleian genealogies, in which the contemporary kings of Dyfed claim descent from Triphun (fl. 450), a great-grandson of Eochaid Allmuir, although the Harleian genealogy itself presents an entirely different version of Triphun's own ancestry in which he descends from a Roman imperial line traced back to St. Helena, whose alleged British origin the genealogist stresses.^[13] This manifest fiction apparently reflects a later attempt to fabricate a more illustrious and/or indigenous lineage for the Dyfed dynasty, especially as other Welsh genealogical material partially confirms the Irish descent of Triphun.^[14] If the relocation of some of the "Déisi" to Dyfed is indeed historical, it is unclear whether it entailed a large-scale tribal migration or merely a dynastic transfer, or both as part of a multi-phase population movement.^[15] However this movement is characterised, scholarship has demonstrated that it cannot have taken place as early as the date implied in The Expulsion of the Déisi (i.e. shortly after the blinding of Cormac mac Airt, traditionally dated AD 265), but must have begun during the second half of the fourth century at the earliest,^[16] while commencement in the sub-Roman period in the early fifth century cannot be excluded.^[17] It is further entirely possible that the historians and genealogists of the Déisi Muman were guilty of lifting these "verified" ancestors, who could have originally belonged to another Irish kindred entirely. Genealogical feats of this kind were famously performed by the Déisi Tuisceart or "Dál gCais".^{[citation needed]}

The term déisi is also virtually interchangeable with another Old Irish term, aithechthúatha (meaning "rent-paying tribes", "vassal communities" or "tributary peoples"). From the 18th century, it had been suggested that this term might be the origin of the Attacotti who are reported attacking Roman Britain in the 360s, although the argument has been doubted on etymological grounds. This argument has recently been reopened, however, by a proposed equation of déisi – aithechthúatha – Attacotti in a late fourth-century context.^[18]

Finally, MacNeill discusses the movements of the Uí Liatháin mentioned above at considerable length, arguing their leadership in the South Irish conquests and founding of the later dynasty of Brycheiniog, figures in the Welsh genealogies matching Uí Liatháin dynasts in the Irish genealogies. He argues any possible settlement of the Déisi would have been subordinate until the ousting of the Uí Liatháin by the sons of Cunedda.^[19]