Hilaria is the feminine of Hilarius — from Latin hilaris (cheerful, joyful). Saint Hilaria (4th c.) was the daughter of Emperor Zeno, who fled an arranged marriage and lived disguised as a monk in the desert of Scetis for years.
Hilaria 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 Hilaria reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.