This graveyard seems to be included in Cantwell's as 'Kilcummin', his map shows it to the north east of Rathdrum town towards Glenealy. His project includes transcripts up to about the 1880s, and includes a number of Cullen/Cullin headstones. One mentions a headstone erected by a James Cullen in memory of his father William died 1859.
Checking the maps it seems to be located to the north of the R752 and across the Railway as you mention - not sure how to access... looks very overgrown on satellite view
There was a copy of Cantwell's for sale on CD at one stage and I think it's now available to search on FindMyPast.
Note not all the headstones Brian Cantwell transcribed in the 1970s are still legible.