I thought of this after reading the Kingsmill massacre when about a dozen Protestants were killed after being asked whether they were Protestant or Catholic when they had no idea what faction the attackers belonged to.
A man "with a pronounced English accent" began talking.[22] He ordered the workers to get out of the bus and to line up facing it with their hands on the roof.[23] He then asked "Who is the Catholic?".[21] The only Catholic was Richard Hughes. His workmates—now fearing that the gunmen were loyalists who had come to kill him—tried to stop him from identifying himself.[22] However, when Hughes stepped forward the gunman told him to "Get down the road and don't look back".[24]
As a followup to OP's question: were there a significant number of non-Catholics/Protestants of any denomination in Northern Ireland at this time? For example, would it have been plausible to claim to have been Jewish or Orthodox, as opposed to atheist? How about, for lack of a better term, "borderline" Christian faiths, such as Mormonism or Christian Science?