A nice article here re U-Boats in the Irish Sea, lightships and some controversial sinkings.
Flanders Flotilla and U-Boat Alley
Of interest are the mine laying maps for Arklow, Dublin and Liverpool coastlines. These are from 1917 when it was becoming harder for U-Boats to operate.
It is strange that no U-Boat operated in this area during the Easter Rising given that re-inforcements from England would have to travel by sea (Liverpool to Kingstown in the case of the Sherwood Foresters for example).