It was of a night

The story of the Prankquean and Jarl van Hoother is based upon a supposedly historical incident which took place in 1575 and involved the Irish pirate-queen Grace O'Malley (Granuaile) and the Earl of Howth Christopher St Lawrence 7th Baron Howth, 17th Lord of Howth (died 1589).

• Samuel Lewis,A Topographical Dictionary of Ireland (London 1837), "Howth":

In 1575, the celebrated Grana Uile or Granuwail, better known as Grace O'Malley, on her return from a visit to Queen Elizabeth, landed here and proceeded to the castle; but indignant at finding the gates closed, as was the custom of the family during dinner-time, she seized the young heir of St. Laurence, then at nurse near the shore, and carried him prisoner to her own castle in Mayo, whence he was not released till after much negotiation, and only upon condition that when the family went to dinner the castle gates should be thrown open, and a cover laid for any stranger that might arrive; a custom which was scrupulously observed during the lifetime of the late Earl.

• Oxford Dictionary of National Biography: "St Lawrence, Christopher":

Christopher St Lawrence had defective eyesight and was known as "The Blind Lord". He was one of the compilers of the Book of Howth, a chronicle of medieval Ireland. He married Elizabeth, daughter of Sir John Plunket of Beaulieu, County Louth, in 1546; they had fourteen children, of whom only six survived to adulthood: Nicholas his successor, Thomas (d. 1600), Leonard (d. 1608), Richard, Mary (married Sir Patrick Barnewall of Turvey) and Margaret (d. 1620). The death of another daughter, thirteen-year-old Jane, was caused by the baron's own hand. In a case before the court of castle chamber in Dublin on 22 May 1579, Lord Howth was charged with having beaten her so cruelly that she died within two days, and also with severely maltreating his wife, Elizabeth (who was confined to bed for two weeks with her injuries), and his butler, who attempted to comfort her. Having heard evidence of the assaults and of the baron's "filthy conversation" and dissolute life with "strange women", the court imposed a fine of £1000. Three years later the court reduced the fine to £500, having heard the baron's plea that he had already been punished to his ‘intolerable charge and hindrance’ by having spent nineteen weeks in prison. Elizabeth Plunket left her husband about 1579, and (probably in the following year) he married Cecily, daughter of Alderman Henry Cusack of Dublin, who, on the baron's demise, wedded first John Barnwell of Monkton, co. Meath, and second, John Finglas of Westpalston, co. Dublin. Lord Howth died on 24 October 1589 and was buried in Howth Abbey.