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.
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.")