Source: Barbara Braswell in email, 251006 - "I also have a Samuel McClement, age 66, dead in 1868 - no source for it, however." This was probably a Civil Record - needs checking in Derry Family History Centre. ALSO, it may not be this Samuel.
It isn't: Name: Samuel McClements Registration District: Strabane Event Type: DEATHS Registration Quarter and Year: 1868 Estimated Birth Year: 1802 Age (at Death): 66 Mother's Maiden Name: Film Number: 101584 Volume Number: 17 Page Number: 210 Digital Folder Number: 4200216 Image Number: 00157