Identity of aircraft that mined the approaches to Holyhead port in July-August 1940?

[Sid Guttridge]

A merchant ship and two navalized fishing vessels were sunk by mines at the approaches to Holyhead port on 22 July and 16 August, 1940.
No U-boats laid mines in the Irish Sea, so it must have been an aircraft. It seems most likely that it was a Ju88.
Does anyone know what Luftwaffe unit laid these mines, what type the mines and aircraft were, and when?

Many thanks

Sid.