Deirdre is the heroine of one of the great Irish legendary cycles — Deirdre of the Sorrows.
Deirdre peaked in the U.S. in the 1960s. Today it is quietly used.
Deirdre 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 Deirdre reduce to 9, The Giver. This is a traditional interpretive system, not a factual claim about the name.