Dymphna is from the Irish Damhnait — little fawn from Irish damh (deer/ox/poet) + diminutive. Dymphna is one of the Irish feminine names — central to traditional Irish Catholic heritage. Irish feminine name reflecting Catholic mental-health heritage.
Dymphna 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 Dymphna reduce to 9, The Giver. This is a traditional interpretive system, not a factual claim about the name.