Open to the public:
Yes, entry is free (Sometimes public access is restricted due to works for example. Check before you visit)
About Glenknock or Cloghogle Portal Tomb: Druid's Altar
The megalith is disturbed, but among the leaning stones it is possible to detect two portal stones at the north-west and a back stone. The capstone is shattered into five pieces and there are many field stones now set at the monument.
Other historic places you can visit: