Gylle de Burgh

https://en.wikipedia.org/wiki/Gylle_de_Burgh

De Burgh was the only daughter of Sir William Liath de Burgh (died 1323) and a sister of Sir Walter Liath de Burgh. Walter was captured and starved to death by his cousin, the Earl of Ulster, in 1332.

Gylle, married to Richard de Manderville, had her husband and his family kill the earl at Carrickfergus on 6 June 1333 in revenge. This murder was directly responsible for the destruction of the great de Burgh lordship of Connacht, and the loss of Ulster to the Gaelic-Irish till the Ulster Plantations of 1610.[1][2]