The Garda Technical Bureau examined the scene at St Mel's Cathedral in Longford yesterday to try to determine the cause of the fire that broke out early on Christmas morning.
An estimated €2m worth of damage was done to the building including the loss of a number of irreplaceable Harry Clarke stained glass windows.
Gardaí refused to say whether were treating the fire as suspicious until the technical examination was complete.
Nobody was injured in the blaze.