Declan is the English form of the Irish Deaglán, a name of uncertain meaning that is sometimes explained as "full of goodness". Saint Declan of Ardmore is said to have brought Christianity to County Waterford in the fifth century, before Saint Patrick. The name was little known in America until the 1990s; it rose from number 545 in 2000 to a top 100 peak of number 95 in 2019 and ranks 139th in 2025.