I am now looking at the events at the beginning of Jan 1918. The airship SSP5 flying from Anglesey had two interractions in this period, the first a fruitless search around midday/ early afternoon on 2nd to the west of Anglesey and then engaged a suspicious object/periscope? on the morning/midday period of 4th with its Lewis Gun.

Based on the sinkings on this site it looks like there were 2 U boats active in the Irish Sea on the 2nd Jan. U91 seems to have headed north? after sinking the Boston City off Pembroke on the 2nd before sinking the Otto near N Ireland on the 4th.

U61 sank Steamer Birchwood on the 3rd Jan 10 miles SE of Enniscorthy, Wexford and then the collier Rose Marie on the 5th 13 nautical miles south east of the North Arklow Lightship and Halberdier on the 6th 27 miles W by N of Bardsey Island.

I suspect the boat on the 2nd may have been U61, but on the fourth could have been either or neither.

The questions I have are:
1 What time did these sinkings occur?
2 What did U91 immediately after the sinking the Boston City?
3 What did U61 do the day before sinking the Birchwood?
4 Was there a Submarine between Anglesey and Dublin or Dublin and Bardsey Island on the morning of the 4th?