Myrna is the anglicized form of the Irish Muirne — from muirn ("beloved, affection, festivity") — the mother of Fionn mac Cumhaill in Irish mythology. Myrna Hansen — Miss USA 1953. Myrna Lim — Filipino singer. Myrna Williams is Myrna Loy's birth name. Saint Murna of Tomgraney — Irish 7th-century abbess.
Myrna 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 Myrna reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.