The Morrígan appears in this Dindshenchas associated with cattle, and not for the first time; she owned a bull-calf of the Donn of Cuailnge, and it was a fight between this bull and Aillil’s bull Finnbennach that brought the Donn of Cuailnge to Medb’s attention (and so begins the Táin, according to some versions).
Mag mBreg, to wit, Brega, the name of Dil’s ox, that is Dil, daughter of Lugh-mannair, who went from the Land of Promise,1 or from the land of Falga,2 with Tulchine, the druid of Conaire the Great, son of Etirscel, son of Mess Buachalla. In the same hour that Dil was born of her mother the cow brought forth the calf named Falga. So the king’s daughter loved the calf beyond the rest of the cattle, for it was born at the same time (that she was); and Tulchine was unable to carry her off until he took the ox with her. The Morrigan was good unto him, and he prayed her to give him that drove so that it might be on Mag nOlgaidi, (which was) the first name of the plain; (and Brega loved that plain). Hence Mag mBreg is (so) called.
Or maybe it was named from Breogan, by whom the plain was cleared. This is truer, and hence the poet said:
Mag Breoga, palm of our origin,
As far as Tuaimm Trebain without weakness.
The eldest of the heroes over seas,
Breoga, overcame Brega.
Stokes, The Bodleian Dindshenchas, Folklore Vol III, 1892, p471.
1 Tír Tairngire.
2 This seems to be a name for the Isle of Man.