Roisin (Róisín in Irish) is the diminutive of Irish rós, meaning "rose" — so the name means "little rose." Róisín Dubh ("Dark Rosaleen" or "Little Black Rose") is a 16th-century Irish poetic personification of Ireland itself, used in political poetry during times of English rule.
Roisin is among the most popular Irish girl's names. Outside Ireland the unusual spelling (Roisin → ROH-sheen, with silent letters following Irish convention) has limited adoption, but the name is rising.
Roisin 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 Roisin reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.