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From what I know, it's probably an Anglicised version of Sean plus ín. I've never seen it before, but Sean (the Irish form of John and original spelling of Shaun) plus the diminutive suffix ín (commonly used in many Irish names) would make sense to me, and it could be written as Shauneen in English.

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With a little bit of research I found Seáinín - it would be pronounced similarly but was almost always used for males in Ireland (because of its equivalence to "Jack" or "Johnny.")