Lewis O'Dempsey, 2nd Viscount Clanmalier (died 1683) was an Irish aristocrat of the seventeenth century.
He was descended from the Gaelic Dempsey sept of Leinster who had secured their lands in Queen's County through the surrender and regrant policy during the Tudor era.
Because of the early death of his father Owny (or Anthony) O'Dempsey, Lewis succeeded his grandfather as the second viscount in 1638.
After the Cromwellian conquest of Ireland he was accordingly dispossessed of his estates in Kings and Queens County by the English republicans.
Her mother Margaret Whitly had married Lewis' grandfather, the 1st Viscount Clanmalier, then an old man, after her first husband's death.