'Mainau' was a German operation by the Abwehr’s Abteilung II to insert an agent, Hauptmann Hermann Görtz, into Éire (Irish Free State up to 1937) by parachute (5/19 May 1940).

Görtz landed successfully after parachuting from a Heinkel He 111 and established contact with his IRA contact 'Agent Hero' (Seamus 'Jim' O’Donovan). Görtz was an Abwehr agent with a chequered career. He had been arrested on suspicion of espionage in the UK on 8 November 1935, sentenced to prison in 1936 and released in February 1939. In the UK he was notorious as 'The Flying Spy'. Görtz was recalled to German service on 1 August 1939 as a reserve second lieutenant, and filled a number of military positions before being assigned, on 19 January 1940, to Hauptmann Dr Theodor von Hippel’s Bau-Lehr-Bataillon 'Brandenburg' zbV 800 under the control of the Abwehr’s Abteilung II.

Despite the agent’s notoriety, the Abwehr allowed itself to be convinced that Görtz was somehow qualified to infiltrate Éire, establish contact with the IRA in the UK, and possibly start a 'revolt' in Northern Ireland. However, the Abwehr’s knowledge about the realities of the political situation in Ireland was very limited.

In preparation for his mission Görtz attempted to discover something of Ireland via the office of Dr Franz Fromme, and he is known to have contacted Francis Stuart, but as far is known the only Irish contact names Görtz carried to Ireland were those of Seamus O’Donovan and Iseult Stuart, the wife of Francis Stuart.

Hauptmann Wilhelm Kaupert was the adjutant at the airfield at Fritzlar near Kassell, and while his primary role was as photographic interpretation officer, he was also tasked with organising Görtz’s flight.

Görtz’s mission was aimed at Northern Ireland, and he was required to use resources gathered in Éire to establish a secure communications link between Ireland and Germany, to consult with the IRA on the prospect of a reconciliation between the Irish Free State and the IRA, to direct the military activities of the IRA against British military targets (specifically naval installations), and to report any incidental items of military importance. Görtz was specifically enjoined against any interference in the politics of Éire or anything which threatened Éire’s neutrality.

Görtz was initially to have been delivered to Éire in April 1940, but this attempt was cancelled as a result of adverse weather. In the interval the IRA’s chief-of-staff, Stephen Hayes, sent the IRA’s 'Kathleen' plan to Germany and Görtz was recalled to Berlin to look it over. Görtz claimed after his capture that the drop zone for the mission was County Tyrone, but this was in all probability a lie intended to indicate that he meant no mischief in neutral Éire. Görtz was not equipped with any of the material which later became standard for agents delivered into Éire, but he was given a radio transmitter, a 9-mm Browning pistol, and invisible ink.

Görtz left Germany on 4 May, and was flown to Éire in an He 111 bomber, from which he jumped in bad weather at 4,920 ft (1500 m). According to his post-war statement he claimed to have landed on a farm outside Trim, but his actual location was Ballivor, County Meath. His radio was lost during the jump, as was the spade with which he was supposed to have buried his parachute. Görtz was in actual fact some 80 miles (130 km) from the house of Mrs Stuart in Laragh, County Wicklow. He walked the entire distance, asking for directions along the way, including at a Garda Siochána station in Poulaphouca, and despite the fact that he was wearing full Luftwaffe parade dress he was not detained.

Görtz reached Mrs Stuart’s house on 9 May. That night Görtz was collected by Seamus O’Donovan, who took him to his house in Shankill, County Dublin. He had successfully made contact with the IRA. On 11 May he was moved by O’Donovan, Stephen Carroll Held, and Patrick McNeela to the home of J. J. O’Neill in Rathmines. During this he handed over to the IRA some $16,5000 in US currency and kept $10,000 for himself. Görtz remained with the O’Neills until at least 19 May, when he was moved to Held’s home. Following this he was to meet with IRA chief-of-staff, Hayes, and remain on the loose in Ireland for 18 months. Held’s home was raided soon after the meeting with Hayes took place, and papers and effects belonging to Görtz, including his parachute, various Luftwaffe identifying badges, German decorations from World War I, and data about Irish harbours and defence installations, were seized.

Görtz remained at liberty, but once it had learned of events from the head of the German legation, Dr Eduard Hempel, the Abwehr considered 'Mainau' to have failed.