Fennell, Gerald (d. c.1666), physician and confederate, was born in Co. Tipperary, where he owned Ballygriffin in the barony of Clanwilliam, and was recorded as taking a lease of Athassal in 1634. He married Ellen, daughter and heir of John O'Mahony of Cloonakeany, in the barony of Ikerrin. He was a close friend of James Butler (qv), 12th earl of Ormond, and also his physician, a relationship that influenced Fennell's political outlook during the upheavals of the 1640s. A regular presence on the supreme council of the Catholic Confederate Association, he was allied with the peace faction, which included Richard Butler (qv), 3rd Viscount Mountgarret, Richard Bellings (qv), and James Tuchet (qv), earl of Castlehaven. Fennell was among those appointed by Patrick Darcy (qv) and Nicholas Plunkett (qv) on 1 November 1642 to formulate the political institutions of the association. His name appears on a letter dated 15 October 1643 protesting loyalty to the Dublin authorities, and he also signed a memorandum on 17 February 1644 regarding negotiations between the confederates and the Spanish crown. Fennell remained in correspondence with Ormond throughout the decade, allegedly furnishing the earl (from 1643 a marquess) with private details regarding the transactions of the confederate supreme councils.
In the summer of 1644 he endeavoured, along with Donough Muskerry (qv), Plunkett, and Geoffrey Browne (qv), to secure the earl's assistance against the Scottish covenanters. As opposition emerged within the association over the terms of a possible settlement, the supreme council sent Fennell and Lucas Dillon (qv) to Waterford to secure clerical support for Ormond's peace proposals. The two men arrived in the city on 8 September 1646, where they initiated negotiations with the papal nuncio, GianBattista Rinuccini (qv). However, the clergy remained opposed and Fennell was arrested, along with Bellings, Muskerry, and others. On 28 February 1647 the assembly appointed Fennell to present Ormond with a new offer, and having met the marquess, he returned to Kilkenny to encourage and persuade the confederates to extend their truce. As divisions within the association sharpened, Rinuccini attempted to veto Fennell's supreme council membership, but to no avail as he was elected once more in November 1647. Fennell is thought to be among those members who supported the Inchiquin truce in May 1648.
With the collapse of the confederate war effort and the Cromwellian campaign, Fennell lost Ballygriffin and the lands inherited by his wife, but received a substantial land grant in Connacht. He is described as physician to Ormond in the Roscommon decree and is likely to have escaped transplantation as members of his profession were in high demand. In 1659 he is recorded as living in High Street, Dublin, where he remained after the restoration. Under the land settlement Ballygriffin was restored to Fennell and he retained his Connacht lands, being described as one constantly in opposition to the nuncio. The 1662 Act of Settlement describes Fennell as one specially meriting restoration, but nevertheless his wife's lands were not returned. There is no evidence that Fennell and his wife had any children: his will, dated 29 November 1663, bequeathed the Connacht estate and Ballygriffin firstly to Ellen, and secondly to his cousin, Gerald Fennell the younger, also a doctor. The will was proved on 10 March 1666.