Rather than recount what has been set forth concerning Cadell Ddyrnllug in detail, here is the splendid summary on this chieftain prepared by P.C. Bartram in his A CLASSICAL WELSH DICTIONARY:
The possible suppression of Vortigern in favor of Catel is also discussed by Rachel Bromwich in her Triads of Britain:
My solution to the "Cadell Problem" is rather simple, but I also feel it is an elegant one. Cadell is a Welsh spelling of the Latin catellus, a word whose meaning is "little/small/young dog/puppy (term of endearment)", according to William Whitaker's WORDS, and "cătellus , i, m., and cătella , ae, f. dim. catulus, canis, I. a little dog, puppy, whelp", taken from the Lewis and Short Dictionary. I would propose that this is a pet-form of Cunorix, 'Hound-king', the son of Cunedda Maquicoline. I have discussed before in detail the Cunorix Stone that was found at Viroconium near Wroxeter, the capital of the ancient Cornovii tribe and of the later kingdon of Powys.
When I asked Dr. Simon Rodway of The University of Wales about the Cadell name being from catellus, he responded as follows:
"This is eminently possible: I’d never thought of this. Otherwise from Celtic *catu- ‘battle’ plus a diminutive suffix –ell."
I would favor Catellus because a certain Cyngen (b. circa 460) was probably his son. A later Cyngen (d. 855) was a son of another Cadell of Powys. According to Dr. Rodway, *Cunogenos gives Cynien. Cyngen must, then, be from *Cunokenos, probably meaning the same thing (cf. gen- ~ cen- in Gaulish), i.e. 'Hound-born.'
If I'm right, then Cunedda's son Cunorix/Cadell/Catellus (likely the Cathlaid the Foreigner who in the Book of Armagh is said to have inherited the church of Trim in Ireland from Foirtchern) was a usurper who either intruded on the Vortigern dynasty for a generation or who founded his own dynasty, effectively ending that of Vortigern's.