When the Irish chieftains sent their ‘Remonstrance’ to Pope John XXII at the time of the Bruce invasion, they included among their indictments of English rule in Ireland the murder of Murchertach MacMurrough, the Irish king of Leinster, and of Art, his brother. The killer they named as ‘Geoffrey de Pencoyt’. This was evidently an event of some moment, to be classed with the more famous murder of Brian Rua O’Brien by Thomas de Clare in 1277 and the slaughter of the O’Conors of Offaly by Peter de Bermingham in 1305. Yet beyond the fact that the murder took place at Arklow on 21 July 1282, very little is known about it. The documents printed below shed new light on the event, suggesting strongly that it was politically motivated and inspired by the Isish justiciar, Stephen Fulbourne. They also furnish tlie crucial missing links between facts that have long been known, but which have not hitherto made coherent sense.