YSTRADWEL (standardized modern form)
Ystradwel is said to be the wife of Coel Hen and mother of Gwawl (see following). She has a calculated date of ca. 360, which places her linguistically in the Brittonic period rather than anything that could yet be called Cumbric. The single source for her existence (although her name occurs in several manuscripts, in a variety of spellings) and the late date of the surviving mentions of her make any interpretation of her name very difficult.
She is mentioned in a text known as "Bonedd yr Arwyr" (see Bartrum EWGT), which occurs in a number of manuscript copies of the late 15th century and later.
Ystradwel -- several manuscripts dating from the late 15th c. and later
Stradweul -- ms. of the early 16th c.
Stratweul -- several mss. dating from the early 16th c. and later
Ystrawavl -- ms. of the mid 16th c.
I am hesitant to connect the name with a transparent reading of its elements. Welsh ystrad "valley" is a borrowing of Latin strata "road", and the presumed deuterotheme in the form -wel is reminiscent of that of names such as Gweirfyl (e.g., 13th c. Wervel, Morfyl (e.g. 13th c. Moruel), Erdudfyl. But these all resolved to an -fy- form, arguing against this interpretation in the present case. It's also hard to balance the arguments for -wel as the earliest surviving version, with -weul as the more widely spread version. The name appears to be a compound of ystrad+wel and as will be discussed for several names, a deuterotheme beginning with "w" has several possible origins. Combining these several uncertainties I'm not comfortable trying to project an early form. (If the protheme is, in fact, the same as the place-name element ystrad, it's also possible that this is a misinterpretation or reinterpretation of a place-name as a personal name -- see the discussion under Gwrygon below.)