Here's a cheeky one:
1916: The Easter Rising leaders aren't executed and the British government doesn't round (as many) people up. Republican sentiment isn't inflamed.
1919-or-so: Ireland finally gets Home Rule, squeaking through on talk of the Glorious Sacrifice Of Irishmen against perfidious Prussia. There's some violence but nothing serious because everyone's sick of war.
1931: George Lansbury, backed with Irish Labour MPs, takes power and decides on disarmament. He'll be forced to change tack by the mid-30s but it's looking too late by then...
1940: The one-two shock of massacres in Scandinavia and bombing of south England cities sees quislings seize power - but the King and enough of the government flee to Dublin, and the RAF and Royal Navy follow. Ireland is now the last stand of the United Kingdom, the last free nation in western Europe, the centre of Empire, the source of resistance aid for the occupied albion; Brits, Irishmen, Commonwealth citizens, and scores of Irish-American volunteers man Spitfires in defence of the island, while the Navy and the Irish people prepare for the inevitable German invasion - Operation Green.
Can Ireland hold out long enough for America to get there?
Who do you think you're kidding, Mister Hitler
You have us all wrong
We're children of a fighting race
That never yet has known disgrace
If you come here, we'll march out, Mister Hitler
You don't know the foe you face