Grainne (Gráinne) is an Irish name possibly meaning "grace" or related to grian (sun). In Irish legend, Gráinne is the woman who runs away with Diarmuid on the night of her wedding to the warrior Fionn mac Cumhaill.
Grainne is widely used in modern Ireland. Grace O'Malley (Gráinne Ní Mháille, c. 1530-1603) was the famous Irish pirate queen.
Grainne 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 Grainne reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.