| Honorific Prefix: | The Right Honourable |
| The Earl of Portarlington | |
| Honorific Suffix: | DL |
| Office: | Member of Parliament for Portarlington |
| Term Start: | 1868 |
| Term End: | 1880 |
| Predecessor: | James Anthony Lawson |
| Successor: | Hon. Bernard FitzPatrick |
| Term Start1: | 1857 |
| Term End1: | 1865 |
| Predecessor1: | Francis Plunkett Dunne |
| Successor1: | James Anthony Lawson |
| Birth Name: | Lionel Seymour William Dawson-Damer |
| Party: | Conservative |
| Parents: | Hon. George Dawson-Damer Mary Georgiana Emma Seymour |
| Children: | 3 |
Lionel Seymour William Dawson-Damer, 4th Earl of Portarlington DL (7 April 1832 – 17 December 1892), known as Lionel Dawson-Damer until 1889, was a British peer and Conservative politician.
Portarlington was the only son of the Hon. George Dawson-Damer and the former Mary Georgiana Emma Seymour.[1] Among his siblings were Lady Georgiana (wife of the 3rd Earl Fortescue) and Lady Constance (wife of Sir John Leslie, 1st Baronet).[2]
His father was a younger son of John Dawson, 1st Earl of Portarlington and Lady Caroline Stuart (a daughter of Prime Minister John Stuart, 3rd Earl of Bute).[3] His maternal grandparents were Lord Hugh Seymour (a younger son of the 1st Marquess of Hertford) and the former Lady Anne Horatia Waldegrave (a daughter of the 2nd Earl Waldegrave).
According to a private letter between Louisa and Eddy Eliot, dated 27 September 1841, their brother's friend "Seymour Damer is gone to school in Liverpool".
Portarlington was returned to Parliament for the Portarlington constituency in 1857, a seat he held until 1865 and again between 1868 and 1880. In 1889 he succeeded his cousin Henry as fourth Earl of Portarlington. However, as this was an Irish peerage he was not allowed to take a seat in the House of Lords.
He was promoted from cornet to lieutenant in the Dorsetshire Yeomanry on 20 April 1858.
Lord Portarlington married the Hon. Harriett Lydia Montagu, eldest daughter and co-heiress of Henry Montagu, 6th Baron Rokeby, on 19 April 1855. Together, they were the parents of several children, including:
He died on 17 December 1892, aged 60, and was succeeded in the earldom by his eldest son, Lionel George. Lady Portarlington died in November 1894.[4]