1) Celtic boy name 2) Irish boy name 
[given name] Declan is an ancient Irish given name anglicized form of an Irish Gaelic saint`s name Declán, also Deaglán or Déaglán, or, in colloquial terms, Det, normally all capitalised when written. Those bearing it include: ...
Declan is a Celtic name for boys. The meaning is `goodness` The name Declan is most commonly given to Scottish boys. (3 times more often than to American boys.)