The pilot of a helicopter was "focused on a bollard" when his blades ripped into a bar and restaurant in Longford.
The owner of the restaurant was caught on video trying to move tables as the helicopter pilot closed in on the "unsuitable" landing space in July 2015.
It then slammed into a timber-clad wall of the Rustic Inn in Abbeyshrule, Co Longford.
The helicopter was destroyed in a matter of seconds and significant damage was caused to the building.
There were three people in the function room on the other side of the wall. One suffered a cut under his eye and another fell as a group of onlookers ran from the crash scene.
Neither the pilot nor his passenger was harmed.
A report was released yesterday by the Air Accident Investigation Unit, which found that the pilot tried to land in an "inappropriate" area that was "very confined".
He was trying to land on a narrow canal bank between the Rustic Inn and the Royal Canal, which was said to have many hazards.
Investigators said that neither Waterways Ireland nor the owner of the Rustic Inn was asked for permission to land.
The owner of the inn said he heard a bang and everyone in the function room ended up on the floor.
A window on the ground floor was broken and glass and debris were strewn across the room.
A radiator in the first floor bedroom was also said to have been torn from its wall mountings.
The pilot said he was "completely aware" where the helicopter's blades were in relation to the building.
He told the investigation team he was "concentrating on a bollard, which I knew was there for tying down the boats on the side of the canal, and I was concentrating on where that was to the skid".
He said he "turned over to look for it and lost concentration".
The tail of the helicopter was severed and landed in the canal.
The helicopter was allowed to fly by the UK's Civil Aviation Authority, which would have allowed it to operate in Ireland for up to 28 days without specific permission. However, because it was an ex-military aircraft, it needed permission.
The report said that the Irish Aviation Authority had told the investigation it had not received a request for the helicopter to operate in Ireland.