ONNEN GREG (standardized modern form)
In one genealogical source, Gwallog ap Lleenog (the brother of Dwywai above) is given a daughter Onnen Greg. This would presumably give her a calculated date ca. 520. Some modern writers have interpreted the two elements as a single name "Onnengreg" while others have interpreted it as a given name and byname.
Onnen grec uerch Wallawc - Bonedd y Seint (Bartrum EWGT), from one 14th c. and some later mss (plus other, more corrupt, variants)
If the second part is taken as a byname and interpreted straightforwardly, it would be a lenited form of creg, the feminine form of cryg, an adjective meaning "hoarse, stammering". This is found as a byname in various records of the 13-15th century. The GPC doesn't offer a derivation of the word, but a straightforward interpretation based on the vowel alternation in the masculine and feminine forms would derive it from Brittonic *cricos / *crica. This would produce an early 6th century *crec. In the radical form, this would likely show up as a Latinized written form Creca with a pronunciation along the lines of ['krEg-@], or in English syllbles, "KREG-eh". At this period, the word would be lenited as a feminine byname, but this lenition would not normally be reflected in the written form. So, used as a woman's byname, the written form would still be Creca but the pronunciation would be ['grEg-@], or "GREG-eh".
The transparent interpretation of the given name as being identical to the common noun onnen "ash tree" seems a bit less likely on a logical basis, but provides the only avenue for pursuing a reconstructed form. The tree name derives from the Celtic root *onno- (found, for example, as Gaulish onno) which occurs in Welsh as a plural onn "ash trees". The suffix -en creates a singular form. I haven't yet been able to track down a discussion of the historic derivation of the singulative suffixes -en (feminine) and -yn (masculine), but their forms suggest evolution from Brittonic *-ina and *-inos respectively. This would suggest reconstructing a Latinized written form Onnena and a pronunciation along the lines of ['on-En-@], or in English syllables "OHN-en-eh" (although the final syllable would be nearly entirely lost at this point).