Interesting that this came up on Navweapons as well.
The way I figure it if you are talking about an airborne attack you have to fly at night just to limit interception by the RAF, so ending up with scattered drops all over the place, will those be able to combine to any massive threat level before UK support arrives? Doubtful and unlike other attacks, you've got limited operational air fields and such so no real chance to bring in follow up air support to defend the invasion.
If you are talking about a sea invasion, first I presume you have to come up from the Bay of Biscay since anything on the channel is going to run head first into the RN, so instead chances are you have troops as sick as dogs who are in unprotected transports, could the IAC Anson's and a few MTB's manage to score some kills when those transports are unloading? As to where, I'm guessing Waterford would be the least worst option given that Cork has it's forts operational but even then as long as the Irish Army held Dublin and Cork long enough for the British Division in NI to move South it''s unlikely that they could win a knock out blow. Meanwhile the RN and RAF would make life hell for them.
For the UK, I'd think it would be best to gamble to let the invasion happen, best case (for them) Dev and a chunk of the Irish politicians ends up dead and the German's get defeated and Ireland joins the war, worst case the Germans get defeated and Ireland joins the war.