ilcoalminer,
I've read a fair amount on medieval warfare and weapons, but don't have anything on the Welsh. However, there is a very nice instructional for building an English longbow in the Traditional Bowyer's Bible, Volume One, Editor Jim Hamm, The Lyons Press, 2000. A friend of mine had an English-style Longbow replica made from ash at one time. I believe it performed well, though it did appear to me to take a little more set than another Oregon yew bow I saw of similar dimensions. Also, the Traditional Bowyer's Bible, Volume Two has a chapter devoted to other ancient European bow designs. The only reference I can find in my small library for primitive bows made of ash describes native American bows. Those made of ash were 5-6 feet in length and drew 60-80 pounds. They were flat-bellied rather than the typical round-bellied English longbow, so you might take that into account if performance is a concern in this project. Anyway, these clues suggest to me that if you were to assume that a Welsh bowyer knew his stuff and how best to make ash perform, it would be a longer bow, perhaps very similar or identical to the English Longbow, unless you made the belly flat instead of round because of the choice of wood.
Anyway, that is the best I have to offer. I'm an amateur and, ironically, the only bows I have built using ash have been children's bows for my son and some of his friends.