Danu is the great mother goddess of the Tuatha Dé Danann, the divine race of Irish myth named for her.
Ancient and nurturing, her name is the wellspring of a whole pantheon.
Danu 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 Danu reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.