Hennessey is an admiral in the English Royal Navy.
During the events of Black Sails: Season 2 Episode 2 X. played by Greg Melvill-Smith
At a tavern in the past, Captain Flint tells Hennessey what Thomas Hamilton told him about changing the world and Hennessey wonders if Thomas is crazy. Hennessey goes to pee and a fight breaks out between Flint and Pickram until Hennessey stops it.
During the events of Black Sails: Season 2 Episode 5 XIII. played by Greg Melvill-Smith
In the past, Flint tells Hennessey Thomas' proposal as they walk to Hennessey's office. They walk into Hennessey's office, where Alfred Hamilton is already waiting and Hennessey tells Flint that Alfred told him what Flint did in Alfred's house. Hennessey tells Flint that he is discharged from the navy and is to leave London.