Now this is what the Oxford Dictionary of English Surnames has got:
Darcey, Darcy, D'Arcy: Norman de Adreci, de Areci 1086; William "Daresci" 1166; Roger Arsi 1173-82; ThomasDarcy 1276. From Arcy (La Manche [= the Channel]). The Irish Darcy derives from JOhn D'Arcy (14th) but is also an anglicizing of Ó Dorchaidhe "descendant of the dark man".
The Oxford Dictionary of First Names adds:
It has always retained a somewhat aristocratic flavour, which has enhanced its popularity as a given name. It is the surname of the Hero of JaneAusten's novel "Pride and Prejudice" (1813).