Declan was presumably born in the early 1700's. He married Corisande, the daughter of Roseclear Harman, at some point. They had at least one daughter, Fianna, in 1770.
Declan is an ancient Irish name, meaning "full of goodness" or "man of prayer". A famous bearer of the name is St Declan, who preached his faith in Ireland shortly before the arrival of St Patrick and established a monastery at Ardmore. St Declan's stone at Ardmore's beach is purported to be the site of many miracles, and crawling beneath it allegedly cures arthritis.
Declan's surname of Morrigan refers to The Morrigan, a famous figure in Celtic Irish mythology. Her name translates to mean "Phantom Queen" or "Great Queen". She is a goddess of fertility, life and death, and battle. The Morrigan could take the form of a crow and sometimes appeared to warriors before a battle, foretelling either victory or impending doom. The Morrigan was sometimes represented by a triad of goddesses, who were all sisters, similar to the neo-pagan Triple Goddess.
Hence, both Declan's first and second names could be considered traditional witch names, but his surname in particular is a very appropriate witch name.