'Fischadler' was a German special forces operation conceived by the Abwehr (intelligence service) and German foreign ministry as an enlargement of 'Wal', and thus based on the idea of delivering special forces of Oberstleutnant Paul Haehling von Lanzenauer’s Lehr-Regiment 'Brandenburg' zbV 800 to Northern Ireland to train and supervise local anti-British forces (June 1942).

The spur for the creation of the plan was the arrival of the first 4,000 US engineering and infantry troops in Northern Ireland on 26 January 1942, and the fears this fact exercised on the mind of Adolf Hitler about the course the war in Europe was likely to take. The planning of 'Fischadler' had it origins in talks between Hitler and Joachim Ribbentrop, the German foreign minister, for the German leader’s primary concern in this region was that the Americans might consider an invasion of Éire, as the Irish Free State (nominally a British dominion) had been known since 1937 and which had declared its neutrality. Such an invasion would mean that the Allies controlled the whole of Ireland. The USA had already violated the neutrality of Iceland and Greenland during the preceding year, and the Germans knew that the Irish prime minister, Eamon de Valera, had been pressured to cede Cork and/or side with the British.

German forces had already considered the occupation of Ireland in 'Grün' (iv), but the German failure to secure strategically decisive victories on the Eastern Front during the summer of 1942 meant that the launching of 'Grün' (iv) in parallel with a revived 'Seelöwe' was not in any way a real probability. 'Fischadler' therefore envisaged the use of volunteers of the standard which would be required for entry into the Lehr-Regiment 'Brandenburg' zbV 800. These would be specially trained in sabotage and the use of British weapons before being delivered to Ireland in the event of any US invasion to train Irish guerrilla forces, Irish Republican Army members, and any Irish army units resisting the invasion.

The Abwehr was to be involved only in planning and the provision of the relevant technical training, while overall control was to be exercised by the foreign service via the RSHA (Reichssicherheitshauptamt, or Reich security main office). The plan and the training schedule were drafted by Walther Schellenburg, the director of Amt VI, while the Abwehr agent Helmut Clissmann was also involved in the training of the agents for some three weeks.

The training was carried out at the Totenkopf barracks in Berlin-Oranienburg, and involved about 100 volunteers selected from various SS troop sections to create SS-Hauptsturmführer van Vessem’s Sonderlehrgang zbV 'Oranienburg'. Each volunteer was trained in the English language, use of British weapons, sabotage and explosives.

The Sonderlehrgang zbV 'Oranienburg' was to be parachuted into Ireland from Focke-Wulf Fw 200 Condor four-engined aircraft after any US invasion of Éire had started. Consideration was also given to the use of the standard members of the 'Brandenburger' unit or, alternatively, prisoners of war of Irish origin currently on the strength of the so-called Irish Brigade of around 10 men at Friesack camp.

In the event 'Fischadler' was not launched as the Americans did not seek to take over Éire. However, the concept was first instance of the involvement of the Sicherheitsdienst, the SS’s intelligence service, in Irish affairs. The IRA in Ireland was not informed of 'Fischadler', although it is known that Frank Ryan, an IRA volunteer located/stranded in Germany and working with the Abwehr, was aware of the mission.