Declan comes from an Irish saint's name whose exact meaning has been lost to time, though it's popularly linked to ideas of goodness or virtue. Even without a settled etymology, centuries of association with a figure known for charitable work have given the name a steady moral undertone.
The qualities of Saturn and Mars are not made better by their Conjunction; being mixed so together, they are confounded, and hurt very much. - William Lilly (1602.-1681.)