Ystradgynlais - Swansea Vale railway / Neath and Brecon
Y in Welsh is sometimes a short i as in bit, and sometimes the indistinct vowel sound schwa = uh roughly, as in currant or the first a in banana, or the o in memory or the first a in separate. Thought I'd better check it up:
- Y - has two different pronunciations. In all but the last syllable of a word it's pronounced as an "uh". When it is in the last syllable of a word it is pronounced as an 'i' - as in English 'is'. For example, the Welsh for mountain is mynydd (mun-ith); the Welsh for mountains is mynyddoedd (mun-uth-oith). Because it no longer occurs in the last syllable, the sound of the second y changes from 'i' to 'u' (also, notice the emphasis moves along to the new penultimate syllable). In single syllable words, the Y is unpredictable!
Your dd in Dolwyddelan would indeed be a voiced "th" as in these, those. You have it in Caerdydd (Cardiff) too.
So Maerdy = Migh-di
Unus - heer for Ynyshir where u represents the schwa sound.
Unusubool for Ynysybwl
Govilon - same as in English I would think
I'm not a Welsh speaker though I can decipher a few common signs. Also North Welsh and South Welsh have somewhat different pronunciations and grammar. So maybe a genuine Welsh speaker can correct me.