Hi Nicole, this maybe a clue -that an dates and names appear to match up:
Down: Upper & Lower Castlereagh - Gravestone Inscriptions, Baronies of Upper and Lower Castlereagh c Marriage Allegations, 1660 Knock Graveyard O. S. 4 Grid Ref. 382729 County: Down Country: Ireland McMechan In memory of Capt. William McMechan, late of Belfast, who departed this life 06 Oct 1809 aged 46 years. Here lieth the body of Elizabeth McMechan, widow of the above named William McMechan, who departed this life on 27 Apr 1846 aged 69 years. Also William McMechan, barrister-at-law, son of the above named, who died 07 Dec 1879 aged 72 years.