I think I remember reading of them at Lathom House and at one of the forts on the Isle of Man too. Not 100% clear but drusty's idea sounds interesting if a little worrying. I like brucka's as I'd thought that perhaps they were catapults of some sort. But it doesn't seem so: There are reference in the Ormond papers -
Warrant to the master of the ordnance : to cause to be delivered to Sir Mathew Appleyard, governor of the fort of Charlemont, for the defence of that fort and use of the garrison there, one demiculverin, one saker, two minions brass and all mounted, four small drakes and one sling piece, without carriages, to be chosen by the said governor out of his majesty's ordnance at Carrickfergus
And
Iron sling pieces.
With a chamber—unserviceable 1
Petard of brasse, fixed—serviceable 3
This from a website selling repro artillery apparently-
link "16th-17th century swivel gun aka port-piece, sling- piece, base-piece, murderer. This cannon is breechloading. Used like a large shotgun. This piece is 6 feet long."
So it looks as if they are some variety of wall-mounted swivel gun.
I don't quite understand the 'Petard of brasse' reference as I thought the petard was a bell-shaped container for explosives that was affxed to an offending door that one needed disposing of. (As someone wrote here before 'operated by a pair of lucky lucky chaps who are about to earn themselves a shiny silver shilling each').