Eithne and Áine are different names.

It so happens that in Donegal pronounciation they sound similar because the Á souunds like EH in Donegal whereas in other rigions it has more like an AW sound and the th tends to be suppressed in Donegal as well.

An Eithne from Connemara is likely to pronounce her name as in ETHNIC whereas in Donegal it will sound more like the ENYA anglacisation, but it still doesn't mean it is the same name as Áine. The name Eithne or Ethna is generally used in both Irish and English, whereas in english Áine is generally translated to Ann(e).

I always thought Wendy was a form of Gwendoline, but I could be wrong.