Current PRONI Reference: VAL/12/B/20/5A (1866 - 1879)
Thomas Duart Plot 18 in Tullymore is crossed and added onto Johns Plot17 but the date is not given so some time between 1866-1879
The sunderland 1871 &1881 census might hold clues see below
freebmds has pre 1891 census death that could link up with 1891 census entry for
Thomas Dewart b~1818 died Jun 1879 Thomas 61 Sunderland 10a 309 freebmd
Agnes Dewart b1809 ireland widow with daughter Mary b 1861 Ireland works in paper mill address Spelter Work Road. 81 census So aneighbour if not granny living nearby etc
Sunderland Dewarts show up in
1891 census combinedwith other freebmds
Street Spelter Work Road, Hendon Ward Sunderland Durham
Robert Dewart b~1843 Gas House Labourer died jun 1915 Sunderland m Barbara XXXXXX b~1842 Durham coincidence Barbara b1856 died 1922 Sunderland
children
Robert Dewart b 1865 Durham Fireman if not coincidence died dec 1922 same time as mother
Eliza Dewart b~1872 Durham
Elizabeth Dewart b~1875 Durham Occupation Paper Sheater
Thomas Dewart b~1876 england if Thomas W b1875 in freebmds
James Dewart b~1879 Durham
John Dewart b~1881 Durham
familysearch.org has Robert Dewart b~1840 Death Date 1869 Event Place Lisburn, Ireland
Registration Quarter and Year 1869 Registration District Lisburn
note Lisburn is really co antrim but as a district registration area was much larger
Ros Davies Co Down has
http://freepages.genealogy.rootsweb.ancestry.com/~rosdavies/SURNAMES/D/DenDey.htm#der.
George DEWART . Hillsborough husband of Ann Cairns; father of John b. 11 Jul 1874 CR
J. DUART . Hillsborough of Tullynore; executor in the will of Henry Scott in 1837 IIW # 59105
J. DEWART . Hillsborough gatekeeper of Downshire Demesne in 1910 POD
Joseph DEWART . Hillsborough a warden at Hillsborough Castle in 1910 POD
William DEWART . Hillsborough a labourer ; convicted at Downpatrick gaol in Lent 1820 of stealing a great coat and received 7 years transportation to Australia; on board ' Almorah', arrived in Sydney 22 Dec 1820; married Honorah Doody/ Dowdy 3 Feb 1832 in Sydney DCM/PD; NWD
William DUART . Hillsborough son of William Duart ; married Jane Neilson 10 Nov 1854 CR
I think thats all I can offer you, hoping others join in