Reiley is of Irish origin, derived from the Gaelic surname Ó Riain, which means "descendant of Rían." Rían was a personal name meaning "little king" or "king's little one." The name gained popularity in English-speaking countries during the 20th century and has since become a unisex given name.
"The name Reiley saw its peak popularity between 1996 and 1999, with over 30 births each year, but has since declined significantly, with fewer than 20 births per year from 2007 onwards."