In 1682 Charles II created him Viscount Mountjoy and Baron Stewart in the Peerage of Ireland.

In 1686 Mountjoy served the Holy League (1684) in Hungary at the Siege of Buda, where he was twice dangerously wounded.

[2] In Dublin he was the centre of a small circle of learned and ingenious men, who had, under his presidency, formed themselves into a Royal Society.

[3] Tyrconnell felt it necessary to replace these troops and decided to raise four new regiments one for each Irish province.

During the Glorious Revolution he stayed loyal to James while most Protestants declared for the Prince of Orange.

However, Rice had secret letters from Tyrconnell for Louis XIV that insinuated that Mountjoy should be arrested.

[9] On his release from the Bastille, he did indeed switch loyalties and joined William's army in Flanders as a General, losing his life at the battle of Steenkerque on 3 August 1692.

Through his daughter Mary, he was the grandfather of two from her first marriage, including Jane Preston (c. 1690–1746) who married Alexander Breckenridge (1686–1743), Col. John Preston (1699–1747), and three more children from her second marriage, including George Forbes, 4th Earl of Granard who married his cousin Letitia, daughter of Arthur Davys of Hampstead.