Siobhan is the Irish form of Joan — from the Hebrew Yochanan, meaning "God's grace." Joan and John share the same Hebrew root.
Siobhan has been used in Ireland for over five centuries. The actress Siobhan Fallon Hogan and the journalist Siobhan O'Connor are prominent bearers.
Siobhan does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Siobhan reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.