'Kathleen' was an Irish Republican Army plan, sometimes known as 'Artus', for the invasion of Northern Ireland authorised by Stephen Hayes, the acting chief-of-staff of the IRA, during 1940 and often confused with the 'Grün' (iv) of the same year for a German invasion of Ireland (early/22 May 1940).

'Kathleen' was conceived at the time when the IRA’s chief-of-staff, Seán Russell, was in the USA aiding the propaganda element of the IRA’s S-Plan sabotage campaign against British interests. At this time Russell was trying to arrange transport to Berlin, and in his absence Hayes was the acting chief-of-staff in Ireland. It was Hayes who authorised the development of a plan for an invasion of Northern Ireland, the defeat of the British there, and the reunification of the island. The plan was drafted by Liam Gaynor, an IRA volunteer, early in 1940, and it was only later that it was decided that the plan should be sent to Germany via courier. Hayes had two reasons for this: he wanted German assistance for IRA operations, and he desired to re-establish IRA links with the Abwehr as a means of securing weapons and financial support.

The courier for transporting the plan to Germany was Stephen Carroll Held, a Dublin businessman of German origins, and he arrived in Germany on 20 April 1940. Held initially contacted the first Abwehr contact in Ireland, Oscar Pfaus, who took Held to Berlin to meet the relevant Abwehr section leader, Kurt Haller. During this meeting Held did not meet the Germans' leading Irish expert, Hermann Görtz, who was preparing to leave for Ireland, but was later introduced to him.

The Abwehr was suspicious of Held and found 'Kathleen' very poorly conceived. After completing his mission of delivering the plan to the Abwehr and inviting the Abwehr to send a representative to Ireland, Held returned to Ireland.

According to Görtz, 'Kathleen' comprised a map on which was the suggestion of a German amphibious landing in the vicinity of Londonderry as the first step in a German military conquest of Northern Ireland with IRA help. The IRA was to be concentrated in County Leitrim on the border facing Lough Erne and Upper Lough Erne. However, the IRA plan gave no thought to how German troops were to be brought to Londonderry, or how control of the sea approaches was to be obtained, or where and how the coast of Northern Ireland was fortified.

The plan was therefore treated as little more than a joke by the Abwehr. The only positive feature the Germans could find in the concept was the implied willingness of the IRA to co-operate with the Germans. It is not known whether or not any serious planning was done for 'Kathleen', although the plan appears to have been widened in scope (by Görtz, or perhaps by the Luftwaffe’s head of airborne operations, Generalleutnant Kurt Student, who presented a similar plan to Hitler in January 1941) to include parachute drops of German troops around Divis mountain and Lisburn in combination with the amphibious assault on Lough Swilly and Magilligan Point.

'Kathleen' reached Germany some two weeks before Görtz was parachuted into Ireland as part of 'Mainau' on 5 May. Before his departure Görtz had been instructed to begin to develop contacts with the IRA, to assess the strength of the IRA and the feasibility of the plan, and to discuss the plan and strength of the IRA with Hayes upon his arrival in Ireland. These discussions were wholly unproductive. Hayes and Görtz began their negotiations on 17 May, Hayes listed the strength of the IRA as, according to Görtz, '5,000 sworn-in members, of whom 1,500 [are in] Northern Ireland. Hayes counts on a further 10,000 northern Irish and 15,000 southern Irish in the case of an armed revolt in Northern Ireland.'

At this meeting Görtz claimed that he directed Hayes to cease operations in Éire and instead to concentrate on operations in Northern Ireland, and that after the meeting he worked out the details of shipping and landing arms in Ireland.

On 22 May 1940 Held’s house was raided and the equipment brought by Görtz to Éire and details of 'Kathleen' were seized. The Garda Siochána (Irish police) waited for Held to arrive and then arrested him. The police also found, in Görtz’s room, a typewriter and file which contained military details about Irish harbours, airfields, bridges, roads, landing places, and the distribution of Irish Defence Forces.

At about the same time, Iseult Stuart, the wife of Francis Stuart, was arrested on suspicion of also harbouring 'Heinrich Brandy', the name under which Görtz was travelling. 'Kathleen' was thus entirely blown, and the Irish authorities immediately sent it to London, where MI.5 further revealed its details to the Royal Ulster Constabulary in Belfast. The plan became the core of joint Irish and British war-gaming in the following year, under the auspices of Plan 'W', the combined British and Irish response to any German invasion.

It seems that 'Kathleen' reappeared, in an expanded form, in Germany during January 1941. In the course of a meeting with Adolf Hitler, General Kurt Student, commander of the XI Fliegerkorps, discussed a plan to take Northern Ireland but leave Éire free from an German intervention. The discussion took place at the Obersalzberg on 1 January as Student was recovering from wounds received during the invasion of the Netherlands in 'Sichelschnitt'. At this point, Hitler was still pondering, albeit only very marginally, an invasion of the UK, and it was within this context that Student suggested a diversionary airborne assault on Northern Ireland to coincide with seaborne landings on the south coast of England. Student suggested a plan whereby dummies dressed as paratroopers would also be dropped to confuse the British, and in a longer discussion the position of the Irish Free State was discussed. Hitler stated that the Irish Free State’s neutrality had to be respected as a neutral Irish Free State was more useful to Germany than a hostile Ireland. Hitler added that he was happy that Ireland had so far remained neutral, and added that it was almost inevitable that a German airborne assault on Northern Ireland would result in small but inadvertent trespassing on the Irish Free state as a result of the transport aircraft straying and dropping their paratroopers in the wrong area.

Student suggested the best date for the operation would be in April 1941 on the 25^th anniversary of the Easter Rising of 1916. His plan was to drop 20,000 paratroopers and deliver 12,000 air-landed troops by night on two areas of Northern Ireland. The first and larger force was to be landed in the triangle between the northern half of Lough Neagh and Divis mountain above Belfast, capturing the RAF fields at Aldergrove, Langford Lodge and Nutts Corner, and at the same time a second and smaller force of paratroopers would be dropped near Lisburn to destroy the British aircraft Long Kesh airfield and cut road and rail links between Belfast and the south. Dummies would meanwhile be dropped over the Mourne and Sperrin mountains to add to the confusion. As dawn arrived, German fighter units would fly in from Brittany and land on the captured airfields.

Student claimed after the end of the war that he believed that the first part of the operation would have been successful, but if the primary seaborne assault on southern England had failed, to avoid surrendering he and his men would have fought through to Éire for internment. What Student did not known, however, was the co-operation the Irish Free State and the UK on Plan 'W', and that any invasion by German forces would have triggered a British movement into the Irish Free State.

Hitler made no decision on the matter, and on the following day Reichsmarschall Hermann Göring informed Student that he was to forget all thoughts off an operation against Northern Ireland, as Hitler had no intention of invading the UK, but instead to concentrate on an operation to take Gibraltar.