Donovan is from the Irish Ó Donndubháin — donn (dark) + dubh (black) + án (diminutive) — descendant of the dark warrior.
Donovan 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 Donovan reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.