Four other people were injured and taken to hospital.
Ireland's Health and Safety Authority has launched an investigation into the incident.
It is expected much of the focus of their inquiries will be on the role played by high winds and bad weather in the area.
Chief Executive of Connacht Gold Aaron Forde said: "The board, management and staff of Connacht Gold Co-op expresses deepest sympathy to the families. Our thoughts and prayers are with them at this time.
"We express our sympathy and our support to the other visitors and employees who suffered injury as a result of the accident.
"We are also providing counselling to those visitors and employees in the store who were affected by this accident.
Connacht Gold opened the store nine months ago.
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