Forster, John (1667–1720), judge and MP, was the eldest son among at least two sons and two daughters of Richard Forster, of Clonshagh, Co. Dublin, militia colonel and later MP for Swords (1692–3), and his wife Anne (née Webber). He entered TCD in 1683 and the Middle Temple in 1685, and was called to the Irish bar. His first official position was counsel to the revenue commissioners, 1699–1710. He was subsequently recorder of Dublin 1701–14, and MP for the city 1703–14. He commanded a militia regiment during the Jacobite invasion scare of 1708 and, as a strong whig, was one of the managers of government business in the house of commons in the session of 1709. Later in 1709 he was appointed solicitor general and, within four months, attorney general (5 January 1710), by the lord lieutenant, Lord Wharton (qv).
In 1710 he was elected speaker of the commons, but when the Irish administration came under the domination of the ultra-tory lord chancellor, Sir Constantine Phipps (qv), he was dropped as attorney general in 1711 and emerged as one of the principal leaders of the opposition. In the 1713 parliament the government's candidate for the speakership was defeated by Alan Brodrick (qv), and Forster was subsequently elected chair of the committee of elections. This was a further setback for the government, since the post carried much influence in adjudicating on contested election returns, an especially important matter after a hard-fought election, which had resulted in closely balanced forces in the commons.
He championed the cause of the city of Dublin during its prolonged dispute with the privy council, led by Phipps, over the acceptability of the aldermen's nominees for the office of mayor. In 1714 he led a delegation of aldermen to London to lobby the ministry directly, and for his efforts his colleagues voted him £500. In that year too he was appointed lord chief justice of the common pleas, a post he held until his death. He was therefore not returned to parliament the following year, but remained an influential figure. He advised the earl of Sunderland, who was appointed lord lieutenant in September 1714 but never came to Ireland, on such issues as the relief of dissenters. Forster died 2 July 1720.
He married first (1695) Rebecca, daughter of Henry Monck of Dublin and Sarah, daughter and heir of Sir Thomas Stanley of Grangegorman; and secondly (date unknown) Dorothy, daughter of George Evans, of Bulgaden Hall, Co. Limerick, MP, and Mary, daughter of John Eyre of Eyre Court, Co. Galway. By his first marriage he left a son and three daughters, and by his second two daughters. A daughter of the first marriage, Anne, married George Berkeley (qv).
He was a kinsman and neighbour of Robert Molesworth (qv) and was related to Alan Brodrick; while his first wife's brother, Sir John Stanley, was chief secretary 1713–14. His brother Nicholas Forster (c.1673–1743) was a fellow of TCD, when, without having enjoyed any previous preferment in the church, he was appointed directly to the bishopric of Killaloe in 1714; he was translated in 1716 to Raphoe, where he spent the remainder of his life.