Modwenna is from the Old Irish mo (my) and the Welsh gwen (white, fair, blessed). Saint Modwenna (d. c. 695) was the Irish-British abbess who founded numerous abbeys across Ireland and England — most famously Burton Abbey on the River Trent. Her cult was popular in medieval England.
Modwenna 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 Modwenna reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.