Rath is the Irish word for fort
Wealthy farmers erected ditches around their farm houses and farm buildings. A fort is a ditch around the early farmsteads. There are several forts in the parish of Rathcoole. The forts are evidence that the people who lived in here two thousand years ago were well organised and worked together.

There is no definite explanation for the name 'Rathcoole'. It could well be Rath Coole meaning 'the rath of Coole', the father of Fionn. Coole may also come from the Irish word for forest, 'coill'. The rath associated with Rathcoole is in a field between Rathcoole and Saggart.
 
Main Source:
"A History of Saggart and Rathcoole Parishes"
by Maeve Mulryan Moloney