Namesakes
Fictional Characters from Television:
2 characters
Declan Costello 1994 Heartbreak High 
Declan Dunn 2000 Mysterious Ways
Notable Musicians:
1 musician
Elvis Costello (a.k.a. Declan) 1954-
Rock and Roll Hall-of-Famers:
1 musician
Elvis Costello (a.k.a. Declan) 2003 Elvis Costello & the Attractions
Saints:
1 saint
Saint Declán of Ardmore ?-4xx