Worse than being a general name for the area Parc Garreg is a name in several different areas of Wales ! I chose the closest grid reference matches , I've seen people on forums be a little touchy otherwise . This particular Parc Garreg I can find only on RCAMHW and no Tre-Maenhir nowhere under any spelling . The RCAMHW refers us to CARN and Dyfed Archaeology but you have to email them to receive further details of specific sites ( which is slightly better than merely listing them ) . They should take a leaf out of RCAHMS where burrowing into CANMAP you can click on individual sites for the full NMRS record and through CANMORE if you know name or some details .