In the incident this is loosely based on, a highwayman named Patrick Fleming was a highwayman so supported and beloved by everyone except the nobility and English landlords, that he couldn't be caught, much like Jesse James. But he let it go to his head, and started robbing and mistreating common folk.

Specifically, he brought his entire gang into taverns, then didn't pay for their lodgings, food, and drink when they left.

On the verse in point, it was actually an innkeeper, not Jenny, who Drew his charges and filled them up with water.

Pub operators commonly kept a supply of opium around, and when a customer got too rowdy and dangerous, he would dose their drink with opium and put them to sleep. Then they could be peacefully removed from the premises.

Having been stiffed numerous times by Fleming he put an end to it by drugging them all, then going to each one of them to pour water into the flash pans of their flintlock pistols.

After he had all of the pan powder wetted, so the guns wouldn't fire, he called the sheriff to come get them.

Fleming and his 14 men were taken to Dublin and hanged on April 24th, 1650.

Fleming had become so hated among the aristocracy that he was hanged... twice.

After hanging him, they took his body down, wrapped it in chains, then hanged him again beside the road, outside of town, leaving the body to rot and be eaten by crows.