Human bones have been found at the site since the early 1930s, according to the records of the National Museum of Ireland, but archaeological material found then was removed in an "ad hoc manner", said archaeologist Mr Malachy Conway.
Bones were found when a garage was built here in the late 1920s. More were found in 1933 when two bungalows were erected next to the garage.
An archaeologist examined them but there was no full-scale excavation, said Mr Rob Goodbody of the Rathmichael Historical Society.
There were more discoveries in 1938 as pipes were being laid at a house. An archaeologist examined these bones but again no large excavation was conducted, he said.
In 1957, the owner of a nearby house found a stone-lined grave when she was digging in her garden, said Mr Goodbody.
An archaeologist was sent out and she recorded what she found for the National Museum, he said.
Mr Goodbody said there were no more records until 1991 when an uprooted tree in a garden revealed human bones.
Archaeologists investigated them but no excavation was carried out. The skeleton was removed.
Next came the current excavation funded by Esso, which began in 1995.
"We knew there was a burial site in the area, but we weren't aware of its size. There are clues that point to the area being an ecclesiastical settlement. There is also a suspected church site to the north-west of the cemetery.
"A nearby area is called Kilbogget: `Kil' or `Cil' is Irish for a small church. Nowadays, that area is on the other side of the road, but boundaries can change," said Mr Goodbody.
The bodies were buried in earth graves and in stone-lined graves. The pattern of the buried bodies may also show the area was a cemetery. Some of the bodies were buried with their heads facing west.
Others, perhaps priests, were buried with their heads facing east, because of the belief that, when the resurrection came, the bodies would rise and the priests would face their congregation, said Mr Conway.
Mr Conway said the sites of the graves may extend past the boundary of the site and Esso will leave these boundaries alone, in case there are other archaeological discoveries.
"It will be down to Duchas and the national monument services to look at full size and nature of the site," he said.