At around 3.45am the fire alarm at Ennis courthouse was activated and alerted the monitoring company.
Gardaí went to investigate and discovered that a window in the family law courtroom had been smashed.
Inside, a bottle and traces of an accelerant, believed to be petrol, were found.
The fire brigade was alerted and two units arrived within minutes – although the fire didn't catch hold.
Gardaí at Ennis are investigating the incident.