On 2 August 1649 his father fought under James Butler, 1st Duke of Ormond at the Battle of Rathmines for the Royalist alliance against the Parliamentarians.
Some time before 1666, Fingall married Margaret MacCarty, second daughter of Donough MacCarthy, 1st Earl of Clancarty and Eleanor Butler.
Luke and Margaret had three sons (of which the elder two died young and are not known by name): —and three daughters: The marriage was a troubled one, giving constant anxiety to friends and family.
In December 1661 Lord Fingall was one of the signatories of the Catholic Remonstrance and presented the document personally to James Butler, 1st Duke of Ormond, for submission to the king.
During the Popish Plot Lady Fingall, who was living apart from her husband in London, was named as a Catholic conspirator, but no action was taken against her.