The origin and meaning of the phrase tongu do dia (toinges mo thúath) has been investigated by R. Ó hUiginn, C. Watkins, J. Koch and T. Sjöblom while K. H. Schmidt has examined its relationship with the Gaulish formula toncnaman toncsiíontío ‘who will swear the oath’. A further example of this type of asseveration occurs in Scéla Moṡauluim where, through imbas forosnai, Finn recites the following quatrains:

Sund Ferchess i fossugud i nEs Máge mámugud do-rrochair caur trénchiṅgeth íar mórgnímaib lúath.

Here [will lie] Ferchess in [his] resting-place, in Ess Máge [will come about] his subjugation. The warrior-champion (Mac Con) has fallen soon after great deeds.

Do-tung dom día tigernmas luige cáich i mbreith[emnas] to-fesar gním guinaitte Mac Con bíth hi-sund.

I swear to my god of lordly beauty the oath of one pronouncing sentence, a deed of slaughter will be avenged, Mac Con was slain here.

In his edition and translation of these verses Meyer translates the first line of the second quatrain as ‘to my lordly god I swear’. The use of an adjective (tigernmas) with the God being sworn to is paralleled by the line ‘dofuṅg-sa do Día uilichumachtach’ from Suidigud Tellaig Temra.