Sullivan is from the Irish Ó Súileabháin — súil (eye) + dubh (dark) + án (diminutive) — descendant of the little dark-eyed one. Sullivan (1858-1918) — male; American heavyweight boxing champion.
Sullivan 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 Sullivan reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.