She was commissioned into the German Imperial Navy on 6 April 1918 as SM UB-123.

[Note 1] She torpedoed and sunk Leinster, a vessel operated by the City of Dublin Steam Packet Company on 10 October 1918, shortly after the new parliamentary based German Government under Max von Baden had asked U.S. President Woodrow Wilson to negotiate an armistice.

Over 500 people perished in the sinking – the greatest single loss of life in the Irish Sea.

Like all Type UB III submarines, UB-123 carried 10 torpedoes and was armed with a 10.5 cm (4.13 in) deck gun.

UB-123 would carry a crew of up to 3 officer and 31 men and had a cruising range of 7,280 nautical miles (13,480 km; 8,380 mi).