The Middle Ages is the setting for most of the story, so namewise I'm looking at names that at least existed at that time. So either names that were in use, or more ancient, more obscure names that would have existed in manuscripts or myth and legend.
With names that "work", I mean names that exist in both languages and are therefore similar, such as Niamh/Nia, or a name that has an equivalent in both Welsh and Irish (another poster suggested
Síne and
Siân, as they are Welsh and Irish translations of
Jane). Or at least a name that wouldn't 'feel' out of place to the characters of the story and as such could be "welshified/gaelified",