O'Neil is an Irish alchemist.
During the events of A Field in England (2013) played by Michael Smiley
Cutler convinces Whitehead, Friend and Jacob to pull on a rope and they pull O'Neil from the ground. O'Neil tells Cutler to bring him his scrying mirror and Whitehead tells O'Neil that he is under arrest for the theft of seven documents stolen from his master. O'Neil tells Whitehead that he sent Cutler to find him so that Whitehead could find the treasure O'Neil thinks is nearby. He brings them to his camp and then tortures Whitehead in his tent. He ties a rope around Whitehead and Whitehead, in a trance runs to a certain spot in the field. O'Neil then has Jacob and Friend dig a hole. O'Neil forces Whitehead to drink wine and Whitehead vomits it out, along with some rocks with runes on them. Friend, Jacob and Whitehead continue to dig the hole and eventually have to put in shoring.
Cutler shoots Friend and is forced to take his place and dig. Cutler hits something with the shovel and tells O'Neil, who tells him there never was an ale house and he just told him that to entice the others. Whitehead spreads his arms out and there is a sudden gust of wind that knocks O'Neil to the ground and breaks his scrying mirror. Cutler tells O'Neil there is no treasure and O'Neil and Whitehead are both frauds. O'Neil then shoots Cutler dead. Friend calls to O'Neil. O'Neil fires his pistol and shoots Jacob in the stomach and Jacob shoots O'Neil in the ankle, which collapses under him. Jacob dies and Friend charges at O'Neil with a spear, but O'Neil shoots him dead. Whitehead then walks up and shoots O'Neil in the back of the head.