What does Casey mean?
From the Irish surname Ó Cathasaigh, whose byname 'cathasach' derives from 'cath' (battle) and is usually glossed 'vigilant, watchful' (in war); a secondary reading is 'noisy'.
What does Darcy mean?
Two distinct origins: as an Irish name it anglicizes O Dorchaidhe, from 'dorcha' meaning 'dark', giving 'dark one'; as an English/Norman name it is the aristocratic d'Arcy, a locational surname meaning 'from Arcy' in Normandy, with no 'dark' sense. Both are genuine; flag which is intended.