SM UC-21 was a German Type UC II minelaying submarine or U-boat in the German Imperial Navy during World War I. The U-boat was ordered on 29 August 1915 and was launched on 1 April 1916. She was commissioned into the German Imperial Navy on 12 September 1916 as SM UC-21.

SS Donegal was a Midland Railway passenger ferry that served in the First World War as an ambulance ship. She was completed in 1904 and sunk by enemy action in April 1917.