ferch Llywelyn Genealogy and ferch Llywelyn Family History Information

Friends, I see from the profile of Ellen ferch Llywellyn that we have a birth date of 1246, but children by her born before that (e.g., Duncan MacDuff b. 1195). I've also see Ellen's birth date listed as 1206, but if her mother is Joan Plantagenet as some state, this doesn't work because Joan's birth date is 1210. Could someone point us all toward an authoritative source for these dates? I am descended from Ellen, I think, so it would be very helpful to pin these things down!

Margaret Elliott