Riley comes from the Irish surname Ó Raghailligh, ultimately from the Old Irish raghaill, meaning "valiant" or "courageous." It was originally a male surname before becoming a unisex given name, then increasingly female.
Riley entered the U.S. The 2015 Pixar film Inside Out — whose heroine is an 11-year-old Riley — kept the name in cultural view for a new generation.
Riley 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 Riley reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.