[nkurz]

Thanks, this confirms that the pronunciation isn't the same at all. "Seoighe" sounds somewhere close to the two syllables "Shoy-Ga", while "Joyce" only has one syllable and a different starting sound. Which still leaves me wondering in what sense these names are equivalent. Was "Joyce" once pronounced differently?

[throwawaymarch5]

"sh" and "J" are closer than you think. They're both pronounced in the back of the throat, so one phoneme can switch to the other given enough time, when moving from one language system to another. The vowel on the end is similar.