Flynn is from the Irish Ó Floinn — descendant of Flann (red, ruddy). Gillian Flynn (born 1971) — American novelist; Gone Girl (2012) sold 20+ million copies worldwide and was adapted into David Fincher's 2014 film with Rosamund Pike — Oscar nomination for Best Actress; Sharp Objects (2006), Dark Places (2009).
Flynn 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 Flynn reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.