If you look at the OS 1:25,000 Map for Carnmenellis G/DC-006, you will see that most of the hill including the trig point which is at the summit is clearly open access land. Therefore, the landowner does not have any right at all to prohibit the access to this trig point.
Craig Y Castell GW/NW-074 is a very similar situation where the whole area is clearly open access land, but the land owner without any rights at all prohibits access. I believe this particular land owner has been reported by hikers on more than 1 occasion. There is even a SOTA Reflector thread about the Craig Y Castell GW/NW-074 “access issue” which is very similar to the Carnmenellis G/DC-006 “access issue”. GW/NW-074 Stop NOW on Craig y Castell!!.
Jimmy M0HGY
G - Association Manager