- "That's the natural way with lakes and castles and fairy mounds... there when you need'em t'be... and, the next mornin', vanished in the mist."
- ―Brendan O'Neal[src]
Brendan O'Neal was a young red-haired green-eyed Irish man who assisted Indiana Jones on a dig in Ireland in 1945, and joined Jones in his quest to find the Spear of Destiny, in a race against the Nazis. He was an expert on Celtic mythology and folklore, an occasional thief of artifacts, and had an eye for women. Despite knowing his first name, Jones usually referred to him by his last name. His mother was an O'Brian, descended from Brian Boru, and she owned a car, which O'Neal borrowed on occasion.
In March 1945, O'Neal was working with Jones in a dig in a mound near New Grange in Ireland. One afternoon, they discovered some six-thousand year old spiral carvings on stone, one of which O'Neal pocketed. As O'Neal was leaving the underground barrow site in the evening (slightly superstitious of the fairy folk), he met a mysterious blond woman, who sought to give a letter to Dr. Jones from his father. Later that night, O'Neal learned of a plot to kill Jones, and snuck into Jones' room at the inn to warn him. Jones, already alert to the Nazi assassination plot, ambushed O'Neal before realizing it was his friend.
Escaping from the inn's second story, O'Neal drove Jones away from his attackers in his mother's car, converted to run on natural gas instead of gasoline. Pursued by Bobby, Pete, and a third Blueshirt in a car chase, O'Neal and Jones leapt to safety just before their car was shot, detonating the gas storage. O'Neal found a horsecart, and he drove Jones to a town on the coast where they could catch a ferry to England. As they were rushing onto the ferryboat, O'Neal got shot by Pete. Jones grabbed his friend and jumped onto the moving ferry. O'Neal remained unhurt, as the shot hit the stolen stone spiral in his pocket, shattering it.
Traveling to Glastonbury, O'Neal and Jones met up with Jones' father, who took them to what he believed was the Holy Thorn on Wearyall Hill, and filled O'Neal in on the history of the Spear of Longinus. At the thorn tree, they were accosted by Edwina Cheltingham, who sought to prevent the elder Jones from plucking the tree. Learning that Nazis were after her tree, Cheltingham befriended O'Neal and the Joneses.
Later, O'Neal helped wrestle Seigfried to the ground when he tried to escape from a meeting with Henry Jones, Sr. at the Chalice Well to find out if Dieterhoffmann had the spear head already. However, O'Neal was forced to let the German youth go when Dieterhoffmann and his goons arrived, with pistols drawn. After being bound by the goons, O'Neal, he heard Dieterhoffmann reveal his plans for the spear, of which he already had the spear head, but was missing the shaft. When Miss Cheltingham arrived, leading a pack of schoolgirls on a botanical walk, O'Neal began to slip out of his ropes. As Indy rushed Dieterhoffmann, the German's pistol went off, shooting the spear tip, which broke into halves, one of which ended up in Seigfriend's hands, and the other in O'Neal's freed hands. O'Neal and the Joneses then ran to freedom.
Reaching the Lady Chapel at Glastonbury Abbey to meet Cheltingham again, O'Neal felt the ruins were spooky. In an underground older chapel, O'Neal, Indy and Henry, saw another mysterious blond woman, who gave them a cryptic message about assembling and guarding the spear. Chasing the figure into the garden, they met Cheltingham, who had brought them a car and a driver, Rebecca Stein. O'Neal thanked the senior botanist with a kiss, who then gave him a sprig of the real thorn. The four took off, pursued again by the Nazis. In the car, Jones took O'Neal's half of the spear tip for safekeeping - which soon after saved Indy's life when a Nazi bullet aimed for his chest, swerved away from the spear tip at the last moment, and into the car's frame.
Impressed with Rebecca's calm getaway driving, O'Neal compared her to the Celtic warrior Nessa, and began to tell her of his Celtic heroic roots in an effort to impress her. On the way to Wales, the car had a flat tire, and O'Neal helped replace it with the spare. However, with the car stopped, they were ambushed by Dieterhoffmann. O'Neal escaped while Jones was tied to a stone and dropped into a lake. As Jones escaped his bonds, O'Neal hauled him out of the water. Eventually Jones and O'Neal reached Holyhead, Wales, where the Nazi group, with Henry and Rebecca as hostages, was attempting to hijack a ferry to Ireland.
Sneaking aboard the ferry, O'Neal attracted attention to himself allowing Henry to escape and Indy to grab Seigfried. Bargaining for his son's life, Dieterhoffmann tossed O'Neal the bundle containing the spear head. When the Nazi tried to turn the tables, O'Neal tossed the bundle into the water and leapt in. Unable to rescue Rebecca, Indy also fled, meeting O'Neal in a small sailboat that his father had appropriated.
Crossing to Ireland in the small boat behind the ferry, O'Neal took credit for secretly swapping the spear bundle he threw in the water for a fake bundle, containing the tip of an ancient Celtic weapon that Indy had used to cut his ropes in his lake escape. However, a German submarine left behind a mine, which detonated, throwing O'Neal overboard. Indy dove in after him and pulled him back to the boat. Eventually, the trio reached Ireland in the rain, using the spear as a compass to guide them to where it wanted to go. O'Neal and the elder Jones headed off to the dig site at New Grange, while Indiana set off to rescue Rebecca and the other half of the spear, still in Nazi hands.
While waiting for Indy to return, O'Neal carved a yew staff for the spear and recounted the Celtic legends of Aredbair, a famous spear, which Henry Jones realized had similarities to the Spear of Longinus, as both were connected to blood and vessels. Recognizing that Aredbair and the Spear of Longinus could be the same, the two followed the words of the mysterious lady from the Glastonbury chapel, and assembled the spear using the sprig of the Holy Thorn, his yew staff, and the half of the spear head they had gotten from Dieterhoffmann in Wales. Indy and Rebecca arrived, but with the fake bundle. Right behind them, Dieterhoffmann, Seigfried and the other Nazis arrived, with the real spear half, which flew from Dieterhoffmann's pocket to join the other half on O'Neal's spear. The spear then flew around the cave, which started collapsing. O'Neal leapt onto one of the stone spirals, which activated a loud shrieking sound, showing that it was the Stone of Fal, and that O'Neal was a true king of Ireland. O'Neal summoned the spear to him, and it stuck him in the upper chest, but did not drive in deeply. Unable to prevent it from penetrating further, O'Neal struggled, but Henry Jones took a hold of the spear, which then illuminated in the dawn's sunlight, striking through a hole in the cave ceiling, lighting up the Stone of Fal at O'Neal's feet. As the spear appeared to bleed for those who believed in its power, the cave began to collapse again. O'Neal was helped out by Rebecca and Henry. Outside, O'Neal noticed that Otto Nehrkorn had escaped, but was confused as to Seigfried's dying ramblings. After the cave finished its convulsions, O'Neal remarked that the cave's collapse was a sign from the "folk below" to keep out of their world.
By August 1945, he had left Ireland, and moved to New York City, where he worked as a bartender. Indiana Jones visited his bar at least once, on August 6, and O'Neal revealed that he had kept a souvenir of their adventures - the tip of the ancient Celtic weapon that Jones had found at the bottom of the lake, and Jones revealed that he had learned that the Spear had been found by American forces taking Nuremberg. As Jones was telling the story, the radio announced the atomic bomb drop by the United States on Japan, reminding O'Neal and Jones of the Spear's power, but were interrupted by a mysterious blond woman - but this time just a union activist wanting a drink at the bar.