"two variants are attested (CGH ed. O’Brien; CGSH ed. Ó Riain), even varying across manuscripts variants, namely Dathal and Daithel. This variation points to original daith ‘quick’ in first position, with dath- due to “restoration” of the more common -al < *-wal. Original meaning thus ‘nimbly valorous’."
Even Patrick Sims-Williams acknowledges that the Irish Dathal may stand behind the place-name Caer Dathal. See
My theory was not well received - not because there was anything wrong with it linguistically, but simply because the Welsh tradition insists this fort was in Arfon. NOT on Mon/Anglesey.
A careful study of the place-names of MATH SON OF MATHONWY, the Fourth Branch of the Mabinogion, plainly indicates that this famous fort is probably either Dinas Dinlle or Caer Seiont/Caernarfon. No other reading is possible. For example, Gwydion and Lleu go for a walk one morning along the shore between Caer Dathal and Abermenai. The only two forts on or near the shore of Arfon opposite Abermenai on Anglesey are those just mentioned.
I now believe I can prove fairly conclusively that I was right - sort of.
When I studied the maps again, I noticed something I had missed before: both Porth(d)aethwy near the northern end of the Menai Strait and Caernarfon near the southern end of the Menai Strait had rivers of the same name. The Afon Cadnants are quite prominent features of both cities. Pasted below are the relevant map sections:
What this tells me is that at some point Caernarfon became mistakenly identified with Caer Dathal/Dindaethwy.
Math son of Mathonwy, "Bear son of the Tribe of the Divine Bear", would appear to have had his chief court at Caernarfon. This makes perfect sense, as the Roman fort of Segontium had surely been the primary center of the region. Thus when we read Caer Dathal in the context of Math's fortress, we must interpret this as a confused reference to Caernarfon/Caer Seiont.
Now, the Dindaethwy fort is generally believed to be Din Silwy or Bwrdd Arthur/"Arthur's Table." So far as I know, Caernarfon has no Arthurian associations. So if Arthur did have relatives at Caer Dathal (even if only in legend), we are probably talking about the real place on Mon.