Spanner crabs are hit and miss,
You need a spanner crab trap and plenty of rope with a large floating balloon like a Pinky not a small float as a small float will be hard to see.
Spanner Crab Traps are rectangular in shape....Crab'n'gear at Clontarf makes and sells direct to public, give them a call , probably cheaper that tackle shops.
Depth various and they like soft mud , usually off South Straddie, It's trial and error to find locations. Rope , good 5mm or 7mm thick...might be anywhere from 60 feet to 90 feet, could be more, could be less...Use your sounder to find the ledges / contours
They could run parallel to the coast or they could run seaward.
Best bait is pillies contained in gutter guard and whole mullet secured to the trap
If you have 4 spanner traps then you can cover bit more area. If nothing found in one spot / drop then move to another location and try there...as said earlier they are hit and miss.
Normally year round but summer is most prolific...there is a closure on taking spanner crabs towards end of year. Check Fisheries website for spanner crab closure dates.
I like a feed of them too but no one has a big enough boat to go outside and have a try for em. Pick a good day to go out sea condition wise.
Hope this helps you.
Bondy99