This gag relies on the understanding that Irish people are a bit, err…slow
An Englishman, Irishman & Scotsman are lined up in front of a firing squad.
The commander asks each in turn if they have any final words before giving the order to shoot.
The Englishman shouts ”volcano erupting!!”
The firing squad turns in fear & he makes good his escape.
The Scotsman yells ”tsunami!!”
Repeated panic sees him escape execution too.
The Irishman cries out ”fire!!”
So they did.
No Irish people were harmed or offended during the course of the joke, to be sure, to be sure.