Mirra is from the Slavic root mir (peace) — also a variant of Myrrh, the resin used as one of the gifts of the Magi.
Mirra 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 Mirra reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.