Hook Standard wet-fly / nymph hook, e.g. Kamasan B170 or Tiemco 3761, #10-16 (a #12 is standard)
Thread Brown (a red thread head is a popular option) 70 denier / 8-0
Materials
Hook Standard wet/nymph hook (Kamasan B170), #10-16 (1)
Thread Brown 70 denier (1 spool)
Tail Brown cock (rooster) hackle fibres, or brown Coq de Leon fibres (6-8 fibres)
Rib Fine copper wire (small/brassie gauge), e.g. UTC copper (1 strand)
Body Hare's ear dubbing (natural mask fur, spiky). This replaces the peacock herl of the original Diawl Bach (small pinch)
Thorax / cheeks (optional) A few strands of peacock herl, or leave as dubbing (1-2 strands)
Beard/throat A few brown hackle fibres (6-8 fibres)
How to tie it
Start the brown thread behind the eye and run it in touching turns to the bend.
At the bend, tie in 6-8 brown cock hackle fibres for the tail, splayed slightly, about half a shank length long.
Catch in one strand of fine copper wire at the bend and leave it hanging; it will become the rib.
Dub a thin, slightly spiky rope of hare's ear fur onto the thread and wind it forward to the 80% point to form a tapered body. The picked-out guard hairs are what make this the 'Hare's Ear' version and add life over the plain original.
Counter-wrap the copper wire forward over the hare's ear body in 4-5 evenly spaced open turns (spiralling the opposite way to your dubbing so it doesn't sink into the fur), then tie off and helicopter/break off the wire.
Tie in a small beard of 6-8 brown hackle fibres under the throat, reaching about to the hook point.
Build a neat head (switch to red thread here if you want the popular red-head trigger), whip-finish, and pick out a few fibres of the hare's ear along the body with a dubbing needle to give it a buggy halo.