Patricia Reidy is a sister of Miriam Reidy, who died in the accident at the Trident hotel in Kinsale.
The two were sharing a room and the alarm was raised at lunchtime yesterday when the women — part of a hen party — failed to check out.
Ms Reidy was found unconscious in the room with her sister, who was pronounced dead at the scene.
Patricia, who was rushed to hospital, was last night understood to be in a critical condition and on a ventilator.
The pair were part of a 30-strong hen night, who, witnesses said, were in “fine spirits” as they visited the bustling seaside town’s pubs and clubs.
Guests who were evacuated from the Trident Hotel following the incident were being accommodated in its sister hotel, Actons, a short distance away, and at other venues in the town.
Meanwhile, a 24-hour help line, 021-477 9300, was set up by the hotel for anyone who might have concerns to call.
The accident has shocked the small tourist town.
Local councillor Kevin Murphy said the town was in shock that someone had died in the incident.
“News of this tragedy has been greeted with horror and disbelief,” he said at the scene yesterday.
Another local councillor, Alan Coleman, said the Trident is one of the town’s flagship hotels.
“In addition to those directly affected by what has happened, it is going to be very distressing for management and staff of the hotel which is one of Kinsale’s flagship hotels,” he said.
“For a town with such a strong focus on tourism, this is a terrible tragedy.”
Dermot Ryan, who is involved in Kinsale tourism, said it was a terrible tragedy for all involved.
“The hotel has a great reputation and is one of the best known in the town. It has been modernised and extended in recent years,” he said.
“It attracts huge business, a lot of visitors from the US, and all over Ireland, and it provides great employment in the town.”