Given the question: Which organization does Sir John Hamilton's servant betray?  Answer the above question based on the context below:  County Cork, Ireland, 1920. Dr. Damien O'Donovan is about to leave his native village to practise medicine in a London hospital. Meanwhile, his brother Teddy commands the local flying column of the Irish Republican Army. After a hurling match, Damien witnesses the summary execution of his friend, Micheál Ó Súilleabháin, by British Black and Tans. Although shaken, Damien rebuffs his friends' entreaties to stay in Ireland and join the IRA, saying that the war is unwinnable. As he is leaving town, Damien witnesses the British Army vainly trying to intimidate a railway personnel for refusing to permit the troops to board. In response, Damien decides to stay and is sworn into Teddy's IRA brigade. After drilling in the mountains, the column raids the village's Royal Irish Constabulary barracks to acquire revolvers, then uses them to assassinate four Auxiliaries. In the aftermath, Anglo-Irish landowner Sir John Hamilton coerces one of his servants, IRA member Chris Reilly, into passing information to the British Army's Intelligence Corps. As a result, the entire brigade is arrested. In their cell, Damien meets the train driver, Dan, a union official who shares Damien's socialist views. Meanwhile, British officers interrogate Teddy, pulling out his fingernails when he refuses to give them the names of IRA members. Johnny Gogan, a British soldier of Irish descent, helps the prisoners escape, but three are left behind. After the actions of Sir John and Chris are revealed to the IRA's intelligence network, both are taken hostage. As Teddy is still recovering, Damien is temporarily placed in command. News arrives that the three remaining IRA prisoners have been tortured and shot. Simultaneously, the brigade receives orders to "execute the spies".
The answer is:
IRA