I think sami and dobfarm have nailed it. For which I am very grateful. My own first guess was also Derrydoon, (thank you Sonas) and I was having trouble making sense of it.
There is a townland named Derry (present parish of Kinawley) close enough to the Old Rossorry Graveyard that it might have been in the parish of Rossorry in the early 1800's.
Thomas Maguire married Lucy Patterson Nov. 24, 1820 in the Cornagee C of I, now the Mullaghdun parish church in the parish of Cleenish. Lucy is shown from the nearby townland of Cleggan.
My current thinking is that Thomas had lived in Derry tl and attended Rossorry parish church, or that the minister or clerk inserted "Derry from Rossorry" to distinguish Derry the townland from Derry the county or the city.
Thomas was born about 1782. The search continues...
Cheers
Carl