Is Síobhan a saint's name

Yes and no!

The name is a cognate of the English Joan, and ultimately derived from the Latin Ioanna and Iohanna, which are in turn from the Greek Iōanna. This Greek name is a feminine form of the Greek Iōannēs,which is in turn a shortened form of the Hebrew Johanan, meaning "God is gracious". So, while there is no saint named Siobhan, there are a number of saints named Joan, Joanna etc.