Henry Dillon, 8th Viscount Dillon was an Irish soldier and politician. In 1689 he sat in the Patriot Parliament. He fought for the Jacobites during the Wiiliamite War, defending Galway against Ginkel and surrendering it in 1691 after a short siege. He obtained the reversal of his father's attainder in 1696 recovering his father's lands.

Viscount Dillon, of Costello-Gallen in the County of Mayo, is a title in the Peerage of Ireland. It was created in 1622 for Theobald Dillon, Lord President of Connaught. The Dillons were a Hiberno-Norman landlord family from the 13th century in a part of County Westmeath called 'Dillon's Country'. His great-grandson, the seventh Viscount, was a supporter of the Catholic King James II of England and was outlawed after the Glorious Revolution. He founded 'Dillon's Regiment' of the Irish Brigade in the French Army, which was supported by the Wild Geese and achieved success at Fontenoy in 1745.