In 1492 two Irish families, the Butlers of Ormonde and the FitzGeralds of Kildare, were involved in a bitter feud. This disagreement centred around the position of Lord Deputy. Both families wanted one of their own to hold the position. In 1492 this tension broke into outright warfare and a small skirmish occured between the two families just outside the city walls.
The Butlers, realising that the fighting was getting out of control, took refuge in the Chapter House of Saint Patrick’s Cathedral. However, the FitzGeralds followed them into the Cathedral and asked them to come out and make peace. The Butlers, afraid that if they did so they would be slaughtered, refused.
As a gesture of good faith the head of the Kildare family, Gerald FitzGerald, ordered that a hole be cut in the door. He then thrust his arm through the door and offered his hand in peace to those on the other side. Upon seeing this, FitzGerald was willing to risk his arm by putting it through the door the Butlers reasoned that he was serious in his intention. They shook hands through the door, the Butlers emerged from the Chapter House and the two families made peace.
Today this door is known as the “Door of Reconciliation” and is on display in the Cathedral’s north transept. This story also lives on in a famous expression in Ireland “To chance your arm”.

http://www.stpatrickscathedral.ie/Chancing-Your-Arm.aspx