Fire services have brought the fire under control at the old Centrepoint Shopping Centre just off the Mountrath Road in Portlaoise.
Two fire units from Portlaoise Station are still at the scene while the Mountmellick unit has returned to base.
The fire was spotted about about 5pm with black plumes of smoke bellowing from the rear of the premises and over a large area.
Major disruptions were experienced on all approach roads as motorists were delayed and caught up in evening rush hour traffic.
This was the second occasion that the fire service have been on the site as they dealt with a smaller fire outbreak earlier today.
To compound matter, at about 6.10pm along the same road an articulated lorry was forced to reverse as it was too high to pass under the Railway Bridge, which again brought traffic to a stand-still along the road for a while.