Salters Grange is a townland in Co Armagh (308 acres). Griffiths Valuation for 1864 lists a Hugh Sleigh in plot 9B(b) in Salters Grange. That is probably your Hugh Sleeth with the name recorded slightly differently. He had a house and garden, indicative of someone who was either retired or a weaver/agricultural labourer.
There is a death registered for a Hugh Sleeth in Armagh city (the registration area included Salters Grange) for 1872, Volume 6, page 58. Estimated year of birth 1807. That’s probably your Hugh.
There were no Sleeth/Sleigh households in Salters Grange in the 1901 census.
Looking at the Sleeth households in Armagh in the 1901 census, of the 40 people listed, the vast majority were Presbyterian, with a couple of Reformed Presbyterians (Covenanters) and a few Church of Ireland. Statistically your ancestors were probably Presbyterian or Reformed Presbyterian (ie 99% likely to be of Scottish descent).
So if you want to find out more about that family I suggest you investigate local Presbyterian records, if they survive. (The Presbyterian Historical Society in Belfast should be able to tell you the churches in the area they may have attended). None of the records will be on-line and you will need to look them up in PRONI, Belfast or at the Presbyterian Historical Society. If you don’t find them there then try Church of Ireland next. It’s the parish of Grange, Co Armagh. Their records start in1780. Copies are held in PRONI.