OOps - here's some information:

Thomas Moore wrote these words in 1811 to an old Irish air Crooghan A Venee. The Regent (later George IV) had been Moore's patron. However, upon ascending the throne the King did not move against Catholic Emancipation in Ireland. Although the song is based on the betrayal of Deirdre and the sons of Usna by Conchubar, the King of Ulster, Moore clearly was criticizing George IV.