Sinn Fein Councillor Cara McShane for Causeway Coast & Glens Borough Council said she was related to the 58-year-old who had been a community midwife.
She also had two children.
"I was very friendly with Deirdre," said Councillor McShane. "Ballycastle is reeling from this tragedy this morning.
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"There would have been a large group of people who, on a daily basis, would have been swimming on the beach.
"This has been an absolute tragedy. There's a real sadness in the area. People are just numb.
"We're all questioning how this could have happened on such a bright, beautiful winter morning. But the sea can be so deceptive."
She added that "the sadness is even more profound given that it is so close to Christmas".
"We will be doing everything, as a family, to support all those impacted," she said.
"Deirdre was 58 and she is such a lovable person, really well liked and loved.
"She is such a pillar of strength for her own two children - and she is such a careful and cautious person who wouldn't have been doing anything that was at all reckless."
She added that Deirdre McShane "has a long term partner and we are all devastated for her immediate family".
"Deirdre worked as a community midwife so she would have been well known, particularly among young families," she added.
"This would have been a regular thing. She would have been going for a swim this morning and then going to work."