Kelly is the anglicized (easier-to-pronounce!) version of the Irish name Ceallach, which likely comes from the Old Irish word for "war", though it has also been said to mean either "church" or "bright-headed". So basically, your choice!
Originally just a surname (and still used as one — especially in Ireland, where it's actually one of the most common surnames in the country!), Kelly started to be used as a first name around the 19th century. At first, it was used strictly for males; at its peak in 1968, it was within the top 100 most popular boys' names. But around the late 1950s, it picked up a lot of traction as a female name too — probably due to actress Grace Kelly, who wed Prince Rainier III of Monaco in 1956. By 1977, it was one of the top 10 most popular girls' names! But though it's far more widely used for girls these days, it's still a wearable choice for boys. We don't know if it will ever regain its former level of popularity, but it has a certain vintage charm as a boy name that is very appealing in today's baby naming landscape.
Notable (male!) Kellys throughout history include actor, dancer, and singer Gene Kelly; pro surfer Kelly Slater; businessman and winner of 'The Apprentice 2' Kelly Perdew; American politicians Kelly Armstrong and Kelly Alexander; actor Kelly Steven Blatz; rapper Machine Gun Kelly; and basketball player Kelly McCarty.
Kelly in quotes:
"I watched Gene Kelly for his smile, for his energy. Vittorio Gassman for his movement. Clark Gable for his mustache. And I watched Lassie who was happy as a dog." — Jean Dujardin
"I have to remind the people who put down East Coast surfing that Kelly Slater is from Florida." — Don Winslow
Meanings
Irish Baby Names Meaning:
In Irish Baby Names the meaning of the name Kelly is: War. Lively. Aggressive. An Irish surname that has only been used as a first name (either gender) in the latter half of the 20th century.