Ireland’s foremost socialist knew that the British Empire and Irish capitalists weren’t the only challenge he and his comrades faced. “In dealing with Ireland,” James Connolly wrote in 1910, “no one can afford to ignore the question of the attitude to the clergy.”

Connolly’s subject of discussion was a 1830s Owenite cooperative that enjoyed brief success, in large part because nearby clergymen didn’t oppose it.

Socialist organizers weren’t usually so lucky. For more than a century — from the mid 1800s until the years after World War II — the Catholic Church was the island’s most consistently reactionary force.