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.
All diurnal nativities are strengthened by the Sun, when well beheld by the fortunes; nocturnal of the Moon, when she is so fortified. If this happen not, yet if good planets be found in angles, the nativity shall be good. - William Lilly (1602.-1681.)