Who is John Hickey from Cathedral and what is their importance?

John Hickey is an IRA veteran. He has been recruited to work with the Fenians. Born in in County Cork, Hickey is believed to be dead. Jack Ferguson, however, recognized him and so did Maureen. Jack is second in command to Flynn in the cathedral operation, and he is described as a radical.... he has no interest in negotiation. When he catches Father Murphy sending a Morse Coded message through the confessional, he takes over the sending and fills the message with disinformation, leading the government to believe they will kill Sir Harold Baxter before dawn.

Hickey hasn't lived in Ireland for over forty years. His father was shot by the British, an event that fueled Hickey's revolutionary fervor. The cathedral is meant to be his final mission.

Hickey doesn't tell Flynn about the governments' offers, or the other things that are going on. Flynn is understandably unhappy when he learns the truth. Hickey seems to want to die.