1) German invasion of Norway and Denmark had the advantage of surprise and was far from English waters. Invasion of Ireland will not have the advantage of surprise and practically goes through English waters.
2) The Kriegsmarine was crippled by the invasion of Norway. When the Sea Lion was being discussed in August 1940 the Kriegsmarine had two antique coastal battleships, one pocket battleship, one heavy cruiser.
3) The RAF would enjoy a tremendous advantage as German shipping was forced to
repeatedly go through the region. By concentrating west-southwest of Cornwall the Luftwaffe can't contest at all!
4) FDR and the interventionists are deeply grateful for the sudden muzzling of the Irish American community, isolationist not so happy.
5) The entire initial landing in Norway consisted of @10,000 men in six locations with a few dozen antique or experimental tanks, no Brits on the spot. The British forces already in Northern Ireland would match that.
CAN the Germans field enough shipping, capable of functioning on the high seas, to land a truly substantial force, say 30,000-40,000 with tanks, artillery and plenty of supplies, something they have no prior experience doing?
6) Germany's invasion fleet of barges was of uncertain seaworthiness in the Channel, going several hundred miles into the Atlantic...
In the event that the Germans actually try this they lose the entire invasion force, their surface fleet is gutted, their merchant fleet depleted and they just helped FDR rearm America and aid Great Britian. Great move.
Fenwick's assumption that the IRA would be as effective against the Gestapo as they were against the British in the 1970s is simply ludicrous, as is the idea that the British in the 1970s were remotely like the Nazis in how they behaved.