HE was born just one minute before his sister but this little boy's almost total failure to develop has baffled the medical world.
Sean O'Shea, now 20 months, has not reached any of the milestones of development associated with infants.
He has never sat up, rolled over or even been able to grasp anything in his tiny hands.
Meanwhile, his twin sister Keelin is walking and talking like any normal toddler their age.
Their parents, John and Collette O'Shea, are frequent visitors to the country's main children's hospitals, and are setting up a charity to help meet Sean's medical expenses as doctors try to establish what's wrong with the little boy.
"They know he's lacking white matter in the brain but they say the longer it goes undiagnosed, the more likely that it's something very rare," Collette said.
"If we knew, good or bad, what's going to happen down the road that would be something."
Collette, from Headford, Killarney, Co Kerry, had a normal pregnancy and her twins were delivered by caesarean section on September 21, 2009 -- her own birthday.
Within four or five weeks, the couple realised all was not as it should be with their son.
"Keelin was developing like a normal baby and becoming more alert and responsive but there was nothing from our little man.
"We took him to Dr Leahy at Kerry General Hospital who immediately knew there was something wrong and sent us straight away to Temple Street, where we've been going since.
"They just can't seem to pinpoint what's wrong with him.
"What's puzzling them is how one of the twins is perfect and one isn't," she said.
Since the twins were born, Collette has been unable to return to work as Sean needs constant care. The couple also have an eight-year-old daughter, Chloe.
"Sean is oxygen dependent at night or any time he's asleep and needs a ventilator. Otherwise, he can stop breathing. He also needs suctioning to clear his airways," she said.
Although his lifeless state rarely varies, Collette is convinced her son is aware of his surroundings and his family.
"There is a definite bond between twins and Keelin always manages to get a smile from him. So does Chloe, who's like a mother hen over him.
"He also responds to a little toy ambulance we got him that makes a noise like an ambulance but if you drop something and make a loud bang he doesn't respond.
"At least we've started looking for answers when he's young, so hopefully they'll be able to find out what's wrong with Sean and put a name on it finally," Collette added.