In June 1940 he landed by U-boat in neutral Ireland on a naval intelligence mission but was captured that same day and sentenced to three years in jail.
With the Battle of Britain about to begin following the fall of France, the arrival of a German spy on the Dingle Peninsula in June 1940 sparked enormous security concerns both in Ireland and the United Kingdom.
Walter Simon was trained at the Abwehr branch in Hamburg, who were responsible for subversive activities against the United Kingdom.
He had collected information on British air defenses – material invaluable to Operation Sea Lion, the planned invasion of Britain.
He gave his name as Karl Anderson, but he was quickly identified as Walter Simon when the Gardaí checked his fingerprints with the British.