Our family farmed in Tircrevan and most recently (most of the 1800s), off Duncrun Road, near Tircrevan Road. We have looked at Dunboe in Articlave (been there several times) and Ballykelly (also a couple of times). No luck. We have the mother of our Caldwell immigrant (Margaret Caldwell), listed on a tomb in St. Aidens. No other Caldwells are there. The brother of the immigrant Caldwell had two spinster daughters and they are buried in Magilligan Presby Church. Their father, Alexander Caldwell, ran the farm for most of the 1800s and died in 1899 - but even his grave is missing. At some point in the 1800s he gave or sold the farm to the Brewsters/Browsters. So we think that all of these Caldwells - all the way through Alexander Caldwell - are buried somewhere that we are not finding them.
I think that the old graveyard you mention is probably a lot older than when we are looking (we know Caldwells were in Magiligan for most of the 1700s until much later and think that they are all related - and hope that finding the missing graveyard will help us solve some puzzles.