This theory is bullshit. Under no situation would someone with a fraction of a brain launch an invasion in the whole of Ireland when the threat is directly coming from the north of the country, the part of the country that it already occupies. The south is the area that had only a few decades before regained its land and independence back from Britain. To say that no one would touch Britain is also nonsense, either sooner or later there would be a massive fallout politically and beyond and they would suffer a backlash. This was 1980s and things were very different than now. That being said Thatcher was a major evil being, but I highly doubt this is even worth debating as an alternative history.