Sinead (Sinéad) is the Irish form of Janet — itself from Jean (the feminine of John), meaning "God is gracious." Used in Ireland for centuries.
Today Sinead is widely used in Ireland and the diaspora.
Sinead 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.
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In Pythagorean numerology the letters of Sinead reduce to 7, The Seeker. This is a traditional interpretive system, not a factual claim about the name.