Marlene,
From the 1831 census, the family was Presbyterian. (That’s confirmed by Matilda marrying in a Presbyterian church too).
Statutory death registration only started in Ireland in 1864. Prior to that only the Church of Ireland kept burial records. Presbyterians normally did not. So if Robert died before 1864 (as seems likely from his absence in Griffiths in 1859) then there’s probably no record. You could try local graveyards for a gravestone perhaps.
I assume you will have noticed Joseph Sloss next door to Robert in 1831. Presumably a relation. Both had families of 7 at that date. Joseph is also gone by 1859 but there is a Matilda, who I suspect is Robert’s widow. She is replaced in the Valuation Revision records by James Sloss in 1868 who looks to be a sibling of David & Matilda. His father was Robert and he too married a McKeown from Curragh in 1856:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1856/09514/5448713.pdf
James looks to have died in 1870. His son Robert John married Nancy Shiels in 1891:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1891/10683/5893330.pdf
This is them in 1901:
http://www.census.nationalarchives.ie/pages/1901/Londonderry/Maghera/Cr…
Probate abstract:
Sloss, Nancy of Crew Maghera county Londonderry widow died 14 April 1962 Probate Londonderry 30 August to Thomas Lytle and Kennedy Shiels farmers. Effects £1859 14s.
The property still appears to be a farm, now on the Drummuck Rd, BT46, and a trawl of the phone book suggests there may be a Sloss living there.
I can’t see a death for Matilda Sloss so suspect she too died before 1864. James’ daughter Mary Ann married in Curran Presbyterian in 1880 so there seems to be a family connection to that church too.
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1880/11040/8038117.pdf
Curren Presbyterian church has no baptism records before 1900 and no marriages before 1845. Maghera has no records before 1843. (The previous Minister Rev Kennedy never kept any). The lack of records means that tracing the family back any further will be very difficult if not impossible. The only hint I can give is that a James Sloss of Crew had a will probated in 1808. Robert & Joseph’s father perhaps? That can only be speculation though. The will itself was destroyed in the 1922 fire but the fact that it existed has survived and is in the PRONI names index.
Being Presbyterian and living in Co. Londonderry points strongly to the family being descendants of Scots who settled in that area in the 1600s.
