@dthomas,
There is no equivalant name as it's a Gaelic-Irish name. Also, Be aware that Hebrew alphabet and fonts would be...ummm,...Greek to you. Seeing that Siobhan in English translation is Irish form of Jehanne, a Norman French variant of JEANNE (Siobhan is another Irish form of Joan meaning "God is gracious)." A popular name in Ireland where the anglicised versions are often used). With that being said, you're still no closer.
A baby's parents who are Jewish choose whatever Hebrew name they like and identify with the most.
http://www.behindthename.com/name/siobhan