Hi,
I'd been searching for the location of Darby'd hole as well and after failing to find it last year, I searched the web, I found the following info:
Darby's Hole - Cove
Map coords:
8 27 59
54 28 13
I checked these out on the ordinance survay and Darby's hole seems to be the small cove just east of Illaunee More (Larger of the small islands) on the north west side of Mullaghmore Head. Haven't tried it yet, hopefully this weekend. :D