Tongu do dia toinges mo thúath

Such formulae are common in early Irish literature, and especially in the heroic sagas, where they are sworn for emphasis when a characters declares they will perform some feat.

On the other hand, Ruairí Ó hUiginn has argued that the oath was a scholarly Christian invention, contrived to suit the pagan background of the Old Irish sagas.

However, the secular narrative literature of early medieval Ireland (and especially the prose sagas) is frequently set in a pagan, heroic past.

Oaths were a feature of legal processes, but were also made in everyday life to emphasise assertions.

[5][6] For example, in the the earliest recension of the Ulster Cycle saga Táin Bó Cúailnge, when the hero Cú Chulainn is told that to kill Fóill, son of Nechtain Scéne, he must kill him with the first blow, he says the following: tongu do dia toinges mo thúath, nocon imbéra-som for Ultu a cles sin dorísse diano tárle mánaís mo phoba Conchobair as mo láim-sea I swear by the god by whom my people swear, he shall not play that trick again on Ulstermen, if once the broad spear of my master Conchobar reach him from my hand.

[11][12] Some have interpreted the oath as a genuine Celtic pagan formula, preserved in early Irish literature.

[5] He argues that the archaic grammatical features in this phrase show a "syntactic imbalance", which would be unexpected in a genuine idiomatic expression.

He considers Ó hUiginn's proposed explanation of the phrase "most unlikely", in view of the absence of documentation for this development, and the small number attestations of the Christian formula.