The bus which is described as a midsized privately owned passenger bus was parked in the Killarney Outlet Centre car park.
Gardai carried out a full technical examination and are awaiting the results.
However they say that currently there is no evidence of accelerants used or any forced entry.
There were also no reports of suspicious activity in the area.
The incident is reported to have occurred at approximately 3am on Saturday.
Investigations are ongoing and Killarney Gardai are appealing for witnesses to contact them.