Reilly is the Irish surname Ó Raghailligh, "descendant of Raghallach", a personal name of uncertain meaning that is sometimes read as "valiant". It is the spelling closest to the Irish original, while Riley became the preferred given-name form in America. Reilly peaked at number 817 for boys in 2001, has since slipped out of the top 1,000 and ranks number 2,708 in 2025, with 48 births.