According to MacLysaght the name Darcy is often spelt D'arcy and this form is historically correct for those descended from Sir John D'Arcy, who was Chief Justice of Ireland in the fourteenth century. These D'Arcys originally from Arci in Normandy did not come as part of the Norman invasion of Ireland in 1169 but arrived somewhat later.
Darcy without an apostrophe is a corruption of the Gaelic name O'Dorchaide which means dark or black but could also mean mysterious or shy. This was first anglicised as O'Dorcey.

Was Tom Lefroy not the Chief Justice of Ireland in later life?