The story dates back before to conscription when recruiting parties were sent out to entice likely young lads to enlist. They were then paid a bounty, but served for something like 20 years (if they lasted that long). For poorer people sometimes it was the only way they could survive, hence a larger proportion of Irish, Scots and Welsh served in the British Army than we would normally expect.

Arthur is wise to all this and refuses. The recruiting sergeant takes this as an insult, but before he and the Corporal can attack them, they beat them up. Many Irish people resented that they were expected to fight in the British Army when then had few freedoms at home.

Hope this helps
Les