Robert 'Alexander' de Irwyn, 4th Laird of Drum

His marriage was arranged to end a violent fued between the Keith's and the Irvine's.

Younger brother Robert carried out his oath and changed his name to Alexander and married his elder brother's fiancée, Elizabeth de Keith. Sometime later he led the delegation which negotiated the release of James I from the hands of the English, for which he was knighted. James V rewarded the sixth Laird of Drum in 1547 for his peace making efforts. Many Irvines also died at the battle of Flodden on Sept. 9, 1513.

From Lundy's thepeerage.com :

He was given the name of Robert de Irwyne at birth. 1

[1] Charles Mosley, editor, Burke's Peerage, Baronetage & Knightage, 107th edition, 3 volumes (Wilmington, Delaware, U.S.A.: Burke's Peerage (Genealogical Books) Ltd, 2003), volume 2, page 2062. Hereinafter cited as Burke's Peerage and Baronetage, 107th edition.