Not a lot known about it. Mate of mine used to be Head of Garvagh High School and was a bit of an amateur historian. He told me that it was built late 19th Century by the Canning family (Lord Garvagh's lot) as a burial place for His Lordship who was a great traveller, much taken by Egyptology and quite fancied his own mummification and subsequent entombment when he snuffed.
However, the local clergy went ballistic when they heard about the plans and after much acrimony, he was planted like the rest of us in the local graveyard. So the pyramid was sealed and remains empty.