I’ll use the words of Irish Technical diver, Stewart Andrews, to tell the story : –

As it turned out, this ‘dark’ and unsettled (deep) season came to a thrilling end with some proper exploration off Donegal. The very best discovery was left until the last dive. A big thanks to “Rosguill” skipper Michael McVeigh and trip leader Rez Soheil with some advice from the deep exploration guru, Barry McGill.

There are only four WWI U-Boats (operational) sunk off the north coast of Ireland –

One is the very large, popular and photogenic U89 in 61 metres, complete with two deck guns.  

Another – U110 sunk the luxurious British liner RMS Amazon and has yet to be found.

Another – U45 which was sunk by HMS submarine D7 – again, yet to be found

That leaves UB124 – one of the Type UBIII Coastal Torpedo Attack Boats which was physically smaller than the others, (distinguished by having just one stern torpedo tube) and having taken over from UB64, finished the attack on the troop carrier RMS Justicia. At over 32,000 tons, the Justicia was the largest ship sunk by U Boat in WWI & the second largest vessel afloat in the world at the time of her sinking
Having sunk Justicia, she herself was destroyed by depth charges on 20 July 1918. This was the only ship she ever sunk.
A very exciting dive indeed!  I hope to talk more about this at TekDive15 in Antwerp in November – in the meantime, here are just two images. The conning tower has been separated from the wreck as a result of the depth charges. The other image shows the single stern torpedo tube with the outer hull missing around the tube.