Well, after the little Nuclear War, and the rise o the White Wyrm in Britain, there was a lot of discussion about "retaking Ireland" by the Wittengamot. Combine this with the mass flow of refugees/dissidents escaping from the remnant UK, and the fact that Ireland has it's own Unionist conflict in the North before, and things get pretty sticky. The White Wyrm invades in '75-ish to support local groups allied with it in the north. and then cover Dublin in Anthrax (they found a lot of it from the old government...). Really, a theme of the idea is that the White Wyrm has no good reason to last, because it is incompetent, dedicated to killing most of it's own population, anti-industrial and faces large opposition from much of the country (including dedicated local opposition in Wales and Scotland), but everything is so messed up it manages to totter on. It's hard to tell from the map, but by the 'early 80s the White Wyrm had already lost control of much of Scotland and Wales a little later, but it was only finally ended when France, Germany, the United States, and the old Colonies got their stuff together in the early '80s and could no longer ignore Britain by '86. The gas attacks were just the White Wyrm's last desperate struggle.
Thanks. Yeah, making icons is difficult. It's hard to balance being understood with the simplicity needed for this scale. And the scales are definitely wrong. I only decreased by 80% for each one (from the largest), which makes it difficult to tell the biggest and second biggest, and the smallest and second smallest. If i'd did it again I'd do more like 66% I think.