There's no single "German" spelling of Shaun. The name Shaun is already of Irish origin. German spellings of names often depend on how the name sounds when pronounced in German. Therefore, depending on the pronunciation intended, several spellings *could* be used, none definitively "the" German spelling. For instance, one might see:
* Shawn: This is a common Anglicized spelling that is often used internationally.
Ultimately, there isn't a standard German equivalent of the name Shaun.