A skilled politician, he believed maintaining the Protestant Ascendancy in Ireland required support from the ruling government in London, whatever its composition.

The most notable included his eldest brother Richard (1612–1698), Katherine (1615–1691), a member of the Hartlib Circle, Mary Rich, Countess of Warwick (1624–1678), and the chemist Robert Boyle (1627–1691).

Boyle entered Trinity College, Dublin in 1630, then moved to London in 1632, where he briefly attended law school at Gray's Inn in March 1636.

[4] He and his elder brother Lewis (1619–1642) spent the next three years traveling in Europe, where they studied languages, mathematics and military theory.

[5] Boyle returned to Ireland on the outbreak of the rebellion in 1641 and fought with his brothers against the Irish rebels at the battle of Liscarroll in September 1642.

Boyle fought with the Parliamentarians until the execution of the king, when he retired altogether from public affairs and took up his residence at Marston in Somerset.

Appointed master of the ordnance, he soon assembled a body of infantry and horse, driving the rebels into Kilkenny, where they surrendered; he induced the Royalist garrison of Cork (English troops with whom he had served earlier in the wars) to defect back to the Parliamentarian side.

By this time Broghill had become a fast friend and follower of Cromwell, whose stern measures in Ireland and support of the English and Protestants were welcomed after the policy of concession to the Irish initiated by Charles I.

He was one of those most in favour of Cromwell's assumption of the royal title,[8] and proposed a union between the Protector's daughter Frances and Charles II.

On 25 November, he was impeached by the House of Commons for "raising of money by his own authority upon his majesty's subjects," but the prorogation of parliament by the king interrupted the proceedings, which were not afterwards renewed.