Gardaí told the Irish Farmers Journal that they were investigating potential criminal damage after the fire caused extensive damage to the shed at Quarrymount, Killeigh on Friday night.
Emergency services were called to the scene after 10pm and the investigation will determine whether the blaze was malicious or not.
Nobody was injured as a result of the fire.
Portlaoise garda station can be contacted with information about the incident on 057-867 4100.
Separately, gardaí in Co Meath have been investigating a fire in a barley field last weekend.
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