Looks like the gravestone might have the incorrect date for James Oliver- "The Will of James Oliver late of Leek County Londonderry Farmer deceased who died 16 December 1875 at Ballymacelvennan was proved at Londonderry by the oaths of the Reverend Thomas Kennedy of Templemoyle (Limavady) Presbyterian Clergyman and Isaac Irwin of Drumaduff (Limavady) Farmer both in said County the Executors." Wording of Will is on PRONI's site and 1st bequest mentions nephew John Irwin.