THE SOLUTION TO THE 'LLYDAW' PROBLEM

There is another possibility for Llydaw which I've barely touched upon in the past.  Only just today did I realize why I had discounted it.  Quite simply, there was no Llydaw/Llydan name in this vicinity.  However, it would appear this place may well have been settled by an eponymous ancestor who belonged to the Ui Liathain.

P.C. Bartram in his A CLASSICAL WELSH DICTIONARY discusses this central Wales location for Llydaw:

Llydaw is the regular Welsh name for Armorica, Brittany. But the name also occurs in Wales,
e.g. Llyn Llydaw in Snowdonia, and there are several cases of the use of the name in the older literature which suggest that it was used for some region in Wales. Just as Devon [Dumnonia] and Cornwall gave their names to Domnonée and Cornouaille in Brittany, so Llydaw [Brittany] seems to have had its duplicate in Britain. John Rhys gave reasons for suggesting that a place named Llydaw was in the region of Llangorse Lake in Brycheiniog (Celtic Folklore, pp.531-6). In the story of the hunting of the boar, Trwyth, in the tale of ‘Culhwch and Olwen’, the men of Llydaw, gwyr Llydaw, are represented as assembling in Ystrad Yw, a cwmwd on the south-east border of Brycheiniog (RM 140). In the Life of St.Padarn (§22) it is said that Caradog Freichfras extended his boundaries of Britannia [Wales] so as to include Letavia. This can be interpreted as extending his kingdom from Radnorshire into Brycheiniog. See s.n. Caradog Freichfras. Again in the Life of St.Illtud (§1) it is said that his father was a man of Letavia, and implies that Illtud was born there. Later we are told that he returned to Letavia to die, but there is a tradition that he was buried in the parish of Defynnog in Brycheiniog. See s.n. Illtud, note to
§1 of the Life.

In fact, two places near Brecon - both ancient funeral momuments - were associated with Illtud's grave (see the attached map).

Illtud's "Graves"

According to Eoin MacNeil ("The Native Place of St. Patrick", in Proceedings of the Royal Irish Academy. 1926. pp. 118–40),

From John Koch's CELTIC CULTURE: A HISTORICAL ENCYCLOPEDIA on the name Brychan:

Brychan seems to have been of Irish descent, and a

possibly cognate Old Irish man’s name Broccán is known,

a diminutive of brocc ‘badger’. The name is attested as

both an ogam Irish and Late Romano-British genitive

BROCAGNI. The reality of Irish settlements in

Brycheiniog (now southern Powys) in the upper Usk

(Wysg) and Neath (Nedd) valleys is demonstrated by

the presence of six ogam inscribed stones in the area,

dating roughly from the 5th and 6th centuries. A longlived

Irish dynasty took hold in Dyfed in south-west

Wales at about this time or somewhat earlier. There

were also Irish settlements at the head of the Neath

valley in Ystradfellte.

I'm guessing that what happened was this: first, a man from the Ui Liathain established a kingdom in south-central Wales.  It was called after him, but it was also mistakenly called 'Llydaw' because the Irish group who founded it was the Ui Liathain. Brycheiniog was to the north and bordered upon the Glywysing where Dinas Powys of Illtud was situated.  Thus Illtud departing 'Llydaw' and ending up at Dinas Powys makes a great deal of sense.

Brycheiniog also bordered on Ercing/Ergyng.

I now feel fairly confident in being a able to state that Illtud's 'Llydaw' was Brycheiniog.