Sean O'Callaghan is one of four informers who went public on their lives in the IRA. Another, Eamon Collins, who produced the best insights on the IRA, was beaten and stabbed to death by former associates on January 21st last.
O'Callaghan and the two others Sean McGartland and Raymond Gilmour, wisely decided to live anonymously outside Ireland and of the three, O'Callaghan was closest to the IRA leadership.
There are questions about some of O'Callaghan's claims and residual suspicions over his role in the murder of another informant. But, given the hostility shown to him by republicans, it is very likely that The Informer contains things the IRA would have preferred kept secret.