[3] Henry Bagenal probably matriculated from Jesus College, Oxford when he was 16 (in 1572 or 1573), but left without taking a degree in order to join his father Sir Nicholas who was then marshal of the army in Ireland.
He was involved in some military disasters, such as a defeat at Glenmalure on 25 August 1580 when Lord Grey led the troops (with Bagenal one of the commanders of the rear) into battle with Fiach McHugh O'Byrne and Viscount Baltinglass in the Wicklow mountain passes.
He sought measures to weaken Hugh O'Neill, Earl of Tyrone, an enhancement of the role of the marshal, and a presidency in Ulster with a shire hall and jail to dispense royal justice.
Whilst on his visit, he wrote to Edward Manners, 3rd Earl of Rutland (a relative of his wife) on 16 September 1586 to ask whether he had a parliamentary borough to spare; he was elected MP for both Grantham and Anglesey and chose the latter.
He succeeded his father as marshal of the army in Ireland and chief commissioner for Ulster in October 1590, and was sworn of the Privy Council.
[citation needed] Brian Friel's play Making History turns largely on the marriage between Bagenal's sister Mabel and Hugh O'Neill.