Up to 70% of cyclists who were treated for a brain injury were not wearing a helmet at the time of their injury.
A study looked at admissions to the National Neurosurgical Centre at Beaumont Hospital in Dublin over a 30-month period.
It found 18 of the 26 cyclists sent for treatment were not wearing helmets, only two were, and the status of six patients was unknown.
The CEO of Acquired Brain Injury Ireland, Barbara O'Connell, is not surprised by the findings.
Ms O'Connell said: "It's great that we now have very clear research indicating the risk of not wearing your helmet, so I'm delighted about that this morning.
"People think when they come off their bicycles that they have been knocked off. For many people it could be a pedestrian stepping out in front of them, it could be a rock on the road or a pothole."
Ms O'Connell said people need to take personal responsibility.
She said: "I think what people need to get the message about is helmets won't stop serious injury but it will reduce the effects afterwards, but you've got to take a common sense response here.
"We don't want to reduce cyclists, it's a healthy sport, but if people could take personal responsibility when they get on their bike in the morning to put a helmet on.
"I mean, to be honest, I've heard people saying to me 'oh I arrive in work with a helmet hairdo'. It could be as simple as that."