Lionel George Henry Seymour Dawson-Damer, 5th Earl of Portarlington JP DL was a British peer and landowner.

Earl of Portarlington is a title in the Peerage of Ireland. It was created in 1785 for John Dawson, 2nd Viscount Carlow, who had earlier represented Portarlington in the Irish House of Commons. He was the son of William Dawson, 1st Viscount Carlow, who had represented Portarlington and Queen's County in the Irish House of Commons, and had been created Baron Dawson, of Dawson's Court in the Queen's County, in 1770, and Viscount Carlow, in the County of Carlow, in 1776. These titles were also in the Peerage of Ireland. The first Earl was succeeded by his eldest son, the second Earl. He was a Colonel in the 23rd Light Dragoons but disappeared the night before the Battle of Waterloo and thus missed the start of the battle. He then attached himself to the 18th Hussars, but after the battle was forced to resign his commission in disgrace, fell into dissipation and 'died in an obscure London slum'.