Okay, after some more research, I *think* I might have pieced this together, but would appreciate some feedback on my methodology.
The Forename 'Darcy' is one that has been repeated in the Sinnamon family in New Zealand until at least the 1950's, and there was a Darcy Sinnamon, born circa 1800 in Co. Armagh, to a Thomas Sinnamon and Mary Anne Wentworth. Prior to this, the forename 'Darcy' had not appeared in the Sinnamon line, however, Mary Anne's father was Darcy Wentworth, as were a number of her Ancestors. Thomas and Mary Anne only had 2 children, one being Darcy, and the other Harriett. From this, I deduced that only this particular Darcy Sinnamon would have children who would continue the 'Darcy' forename in future generations, as it was a family name from his Mother, Mary Anne Wentworth, rather than from his the Sinnamon side.
As a result, I believe that Darcy Sinnamon, born circa 1800, is in fact Hugh Lockhart Sinnamon's father.
Any feedback on this would be most appreciated!