O'Brien, Charles (1670?–1706), 5th Viscount Clare , army officer, was the second son of Daniel O'Brien (qv) (c.1630–1690), 3rd Viscount Clare, of Carrigaholt, Co. Clare, and his wife, Philadelphia Lennard (b. 1644, d. in or after 1699), daughter of Francis Lennard, 14th Baron Dacre. He was a great-grandson of Sir Daniel O'Brien (c.1580–1663), created lst Viscount Clare in 1662, and brother of the 4th viscount, Daniel O'Brien (d. 1693). In 1689–90 he commanded one of the infantry regiments raised by his father for James II (qv), and led a regiment of cavalry at the second siege of Limerick in 1691. After the surrender of Limerick, he went into exile in France in 1692, initially serving as a captain in James II's bodyguard and later in the queen's regiment of foot dragoons. O'Brien fought with the dragoons at the battle of Marsaglia in Italy on 4 October 1693. He succeeded to the title as 5th Viscount Clare when his brother died as a result of wounds received during the same battle. He was also appointed colonel of the queen's regiment after its commanding officer, Colonel O'Connell, was killed in action at Marsaglia.
On 8 April 1696 Clare became colonel of the Clare regiment that his father had raised and sent into French service in 1690 under the command of his late brother, Daniel. In the same year he took part in the siege of Valenza in Lombardy, and in 1697 he campaigned with the French army on the Meuse. While Clare's Irish regiments were at all times in French service, at the outbreak of the war of Spanish succession, Clare fought with the Bavarian army when the elector of Bavaria became an ally of Louis XIV. He was promoted to brigadier general on 2 April 1703, and the following September his regiment played a major role in the French victory at Hochstadt. He was appointed major general in 1704, with overall command of a brigade of the Irish regiments of Clare, Dorrington, and Lee. The brigade fought at the battle of Blenheim on 13 August 1704, resulting in a massive defeat for the French. Although twenty-seven battalions and twelve squadrons of French dragoons became prisoners of war, Clare extricated himself from the village of Oberklau and successfully led his three regiments in an unbroken retreat to the Rhine. On 2 October 1704 he was appointed maréchal-de-camp. After the defeat at Blenheim, the French campaign moved to Flanders; Clare's regiment was heavily engaged at the battle of Ramillies on 23 May 1706, where he was severely wounded. He died a few days later at Brussels and was buried at the Church of the Holy Cross at Louvain (Leuven).
Clare married, at Saint-Germain on 9 January 1697, Charlotte (d. in or after 1714), eldest daughter of Henry Bulkeley, master of the household to Charles II and James II. They had two children: a daughter Laura (b. 1697) and a son Charles (b. 1699), who became 6th Viscount Clare on the death of his father. On 1 July 1703, although he was not yet five years old, the young boy was enrolled as a captain in the Clare regiment. When Louis XIV conferred the command of the Clare regiment on Lt.-Col. Murrough O'Brien of Carrigogunnel, he stipulated that an annual sum of 6,000 livres should be appropriated out of regimental pay for the son of its former commander. The 6th Viscount Clare began his military career in Spain in 1719 and, on the death of Murrough O'Brien, he was commissioned as commander-in-chief of the Clare regiment in 1720; he commanded the regiment at the battle of Fontenoy in 1745.