Angela Harwood, coordinator of the Special Olympics, said she was instructing a group of swimmers at the centre when she saw the woman collapse on the ground beside the pool.
She said a group of people had gathered around her trying to help.
"We could see that something was happening so we concentrated on getting our people out of the water and away from it.
"I wanted to see if I could help but you get too many people. You just don't want everybody around so I just got all our kids away."
Shortly after the incident the entire facility, including the gym, was closed to the public.
A regular swimmer, Annalise Brady, said when she arrived at the pool about 7.40pm, a small crowd of people leaving the pool could be seen outside.
When she tried to enter the facility, a pool employee told her the pool was closed due to an accident, she said.
"I wasn't sure what was going on. Everyone was coming out of the pool."
Another swimmer said she found the incident quite shocking.
Mr McGee said the council, which owns the facility, would be working with staff, contractors and anyone at the pool when the incident happened in the coming days to offer any support required.
"Our thoughts are very much with the instructor and her family at this time, as well as those who were at the pool when the incident occurred."
