Police say last month's fire at St Paul's Trinity Church in Cashel Street was deliberately lit.
They want to speak to witnesses who may have seen anyone hauling a mattress or other materials along Madras or Cashel street before the fire on August 5.
The fire was discovered at 3.27am but the Fire Service believe it had been going for at least half an hour before they arrived.
"We believe it has been deliberately lit," says Constable Matt Reynolds.
"There were also cans lying around the mattress."
Constable Reynolds wants to speak to anyone who may have any information on 363 7970.
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