The medieval Irish narrative The Expulsion of the Déisi attributes the kingdom's founding to Eochaid, son of Artchorp, who was forced across the Irish sea in the 5th century; his descendants founded the line of the kings of Dyfed down to "Tualodor mac Rígin" (Tudor map Regin).

Dyfed eventually comprised at least seven cantrefi: Cemais, Deugleddyf, Emlyn, Cantref Gwarthaf, Pebidiog, Penfro and Rhos, with an approximate area of about 2,284 square kilometres (882 sq mi).

[9] Historical attempts have been made to cast the relationship as one as a confederation of Christian unity on the isle of Britain, under the leadership of Alfred, against the heathen Danes.

"The recognition by Welsh rulers that the king of England had claims upon them would be a central fact in the subsequent political history of Wales," according to Davies.

[9] Through his marriage to Elen, Hywel incorporated Dyfed into an enlarged realm to be known as Deheubarth, meaning the "south part", and later went on to conquer Powys and Gwynedd.

Hwyel's grandson Maredudd ab Owain recreated the kingdom of his grandfather, but his rule was beset with increasing Viking raids during the latter part of the 10th century.