Northern Ireland - Any Auxiliary Units? Any Countermeasures Against German or Irish Infiltration?
Certainly the main thrust of Operation Sealion would be England, Scotland, Wales. However, what were the defensive and counterintelligence measures for Northern Ireland?
Eire was officially Neutral, but there was long history of border warfare and sectarian discord. It could have been a route for infiltration. Presumably, there was some secret accord between London and Dublin that as long as things remained status quo, and Irish enlistment in British forces would be tolerated, the Republic would not be occupied.
As the American presence increased and Operation Sealion receded, there would have been little likelihood of invasion. Nonetheless, there were many targets for sabotage, either by Germans or IRA, such as the Harland & Wolf Shipyards, etc.
Officially, I suppose, it would be said that it was all Special Branch, Royal Ulster Constabulary and its Specials, the Intelligence services, and possibly some Home Guard. But, . . . ? Certainly an essential but weak point further from the action but subject to infiltration or subversion. What is known?