The Oranmore megalith.
This monument consists of huge rock by the side of the old Dublin-Galway road, just west of Oranmore village and north of the road. This may be some kind of portal dolmen monument, or more likely a boulder burial, a specific sub-type of monument, as the huge rock rests on a number of smaller uprights or mini-orthostats.
Boulder burials are a rude type of dolmen, thought to date from the Bronze age, the period after the great neolithic colonization of Ireland. Perhaps they are later copies of earlier dolmen type structures. A cremated burial would presumably have been placed under the huge capstone. Here is an excellent article on Boulder Burials from Roaringwater Journal.
Megalithic monuments are not common in this part of the west of Ireland, which must not have been suitable for neolithic farming. The ruined remains of a portal dolmen can be found, with some difficulty, by the east bank of the River Corrib between Galway and Menlo, not far from Oranmore. The next closest monuments are those around Cong at the inner end of Lough Corrib, or in the Burren down to the south across Galway Bay.