The Irish Olympian said that transgender people have an “unfair advantage in sport.”
The comments come after a US transgender swimmer, Lia Thomas, won a 400 yard freestyle relay.
Lia previously competed on the University of Pennsylvania’s men’s team before transitioning and moving to the women’s team.
Speaking to Newstalk’s The Hard Shoulder after penning an article on the topic for The Irish Times, the Cork runner said that she does not believe that ‘transgender women’ should be allowed to compete against ‘biological women’.
"It's on the fringes of sport at the moment - but it's something that a lot of people are wondering about, if it's something that should be allowed or not in women's sport,” she said.
"That a male who becomes a transgender woman, are they allowed to then compete against biological women?
"With the science behind it all, it just doesn't make sense that they should be allowed to do that."
She also claimed that trans athletes have an “unfair advantage.”
"Without a doubt they have an unfair advantage... they take hormones to reduce their testosterone levels, but that's just one factor.
"There's so many other factors in male adults compared to female adults that make them stronger.
"They're at a different level of ability compared to women, that you can't compete.
"When you've already separated the sports into men's and women's sports - then to allow a variable in that the balance is then thrown off - it just doesn't make sense for that to happen, without considering the women who are already in the sport who have been competing and training... throughout their life.
"All of a sudden they've got to face somebody who's just decided - they obviously decide for very personal reasons why they change - but then to also take part in sport, which is for women."
Her comments prompted a huge online reaction with a split of people both agreeing and disagreeing with her on the divisive topic.
“Reading comments in support of Sonia O’Sullivan’s article is a grim start to the day that I wish I hadn’t inflicted on myself. Data, research & open discussion matters but a blanket discrimination isn’t the answer,” one person said.
“Astounding ignorance from Sonia O’Sullivan, an Irish athlete who I have respect for. To see her continue to push harmful rhetoric that trans athletes are a dangerous problem to be ‘dealt with’ is unbelievably disappointing,” added another.
Meanwhile a third said: “The article by Sonia O'Sullivan hurt me to the very core of my being. I used to love watching her race, she was inspirational. Today I find myself going between crying, despair and anger. The one positive is seeing the support from our wonderful allies. Thank you so much.”
Transgender athlete Joanna Harper, who previously worked with the International Olympic Committee (IOC) is a researcher and a medical physicist at Britain's Loughborough University.
In 2021 told Outsports.com: “For those who suggest trans women have advantages: we allow advantages in sport, but what we don’t allow is overwhelming advantages,” she said.
“Trans women also have disadvantages in sport. Our larger bodies are being powered by reduced muscle mass and reduced aerobic capacity, and can lead to disadvantages in quickness, recovery and a number of other factors.
“The bottom line is, we can have meaningful competition between trans women and cis women. From my point of the view, the data looks favourable toward trans women being allowed to compete in women’s sports.”