hmmm
The 53rd (Welsh) Infantry Division was based in NI in 1940, along with whatever forces the Unionists had which id assume were close to Civil War levels or close at least.
In terms of Aircraft I don't think there was any, or less than a handful anyway, considering the lack of AA guns in Belfast when it got bombed. According to wikipedia and the DFI 1940 documents page the British planned to send 3 Hurricane squadrons plus 2 Fairey Battle squadrons.
We had
- 4 Gloster Gladiators
- 6 Hawker Hinds
- 6 Avro Ansens
There might have been a Hurricane (there was one operational by the end of 1940 but not sure when it crashed and was repaired).
Id assume a few destroyers around or near Irish waters at the time, the Royals Marines were suppose to land a battalion in Wexford via destroyer on the day any German attack as well - the British thought they'd catch at least 1/3 - 1/2 of any naval expedition even with Sealion occurring owing to no Luftwaffe support for a German force headed for Ireland so that kinda reinforces the idea there was a small force around the Irish sea.
The big thing to me is that Case Green has to occur either before or at the same time as Operation Sealion purely because the objective was to pin down the British forces in Ulster and draw British units away from the "decisive" battle in the UK. Id suspect Sealion will crumble after a few days as the Germans can't support it logistically when the RN enters the channel in force.
According to wiki;
- to draw off British Army troops stationed in Northern Ireland who might otherwise be sent to aid the defence of Britain,
- deny Ireland as a staging point/refuge to British troops,
- provide a staging post to Luftwaffe forces in subduing northern Britain.
from der Fuhrer
"..a landing in Ireland can be attempted only if Ireland requests help. For the present our envoy [assumed to be Dr.
Eduard Hempel of the German Legation] must ascertain whether De Valera desires support and whether he wishes to have his military equipment supplemented by captured British war material (guns and ammunition), which could be sent to him in independent ships. Ireland is important to the Commander in Chief, Air, [Göring] as his base for attacks on the north-west ports of Britain, although weather conditions must be investigated.
The occupation of Ireland might lead to the end of the war.