The turning point came when the code-breakers at Bletchley Park managed to read U-Boat signals which were supposed to be highly secret: due to advanced encryption techniques.
Breakthroughs took place after U-110 was captured, far west of the UK, on 9 May 1941. The capture of this submarine was one of the biggest secrets of the war for seven months.
http://en.wikipedia.org/wiki/German_submarine_U-110_(1940)
U-559 was another important capture, not so far north, off the coast of Egypt in October 1942. This allowed the Bletchley Park experts to read the ciphers for several weeks, and to decipher U-boat coded messages thereafter.
The "wolfpacks" of submarines found the Irish Sea too dangerous and tended to operate further out, in the Atlantic Ocean itself. Despite the dangers, occasional forays were made into the area: an example being U-246, which sank just a short distance south of the Isle of Man. No-one knows why it sank, so the demise of this submarine remains enigmatic.
Some papers that were possibly from this submarine were rumoured to have been washed up on the shore at Port Grenaugh. Allegedly, part of a coded message was found:
XSLI LBYT EVQD ICUH RLSX HWFZ PXGB VGQL MNXU FCIK UPLR CLDF LZCL BXJF PLFC GYJC TI
along with some hand-written jottings:
TYPE: 3
Umkehrwalze: B
Ringstellung: ABC
Walzenlage:
Grundstellung: .
Can you make anything of the message? To discourage muggle finds I've omitted the "Walzenlage" and "Grundstellung". But to make the deciphering easy, the required settings will be found in open view at N54° 13.xxx W004° 25.xxx.
You can validate your puzzle solution with certitude.