These are the arms of Rowan-Hamilton of Killileigh, County Down. In the nineteenth century a member of the family was an Irish patriot and a friend of Wolfe Tone. He was condemned to death by the English government, but escaping to America, he founded an American branch of the family. From its style, the stamp is of the twentieth century, and may well be American.
Sir William Rowan Hamilton (1805-1865) was Professor of Astronomy at Trinity College, Dublin, and an outstanding mathematician. He was an acquaintance of William Wordsworth with whom he corresponded. There was a sale at Sotheby’s 21-22 July 1983.