Kelly is the anglicized Irish surname Ó Ceallaigh — from the Old Irish cellach ("warrior, strife" or possibly "frequenting churches"). Kelly Slater is male (surfer).
Subject of countless American Idol retrospectives.
Kelly 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 Kelly reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.