I hope this helps some. It is from a book I have called "Celtic Baby Names" by Judy Sierra.
"Declan (DEK-lawn) Declan was the name of a 6th-century saint who founded the monastery of Ardmore in County Waterford. His fest day is July 24. Currently a popular name."

I actually met someone with this name here in Seattle. He had moved here a few years ago from Ireland. I think it is a great name.