Two of the Irish people -- including a 10-year-old girl -- have been diagnosed with narcolepsy, a rare condition where a person falls asleep suddenly and unexpectedly.
The European Medicines Agency (EMA), which is conducting the investigation, said yesterday it was looking at 162 cases across Europe to find out whether there was any link between the swine flu jab and the condition.
Hilary Dowdall, from Stamullen, Co Meath, told how her daughter Chloe was diagnosed with narcolepsy in November, six months after getting the injection.
"I'm not jumping to any conclusions about a link but I am anxiously awaiting the results of the investigation," she said.
Chloe, a pupil at St Fiachra's school, Beaumont, Dublin, was an active, healthy youngster but six weeks after getting the swine flu jab, she developed certain symptoms, which included collapsing after laughing.
"She started to sleep excessively during the day and I thought initially she was just very tired," said Ms Dowdall.
Ms Dowdall embarked on a search for a diagnosis, with blood tests and brain scans coming up clear. Eventually, Chloe was referred to Temple Street Children's Hospital.
An appointment was then arranged with a sleep disorder specialist in the Mater Private Hospital in Dublin.
Chloe underwent a sleep test in November and narcolepsy was confirmed. She is now on medication that keeps her awake at school during the day.
The EMA said its initial review found "no causal link" between the vaccine and narcolepsy.
However, in recent weeks, it began a more intense probe after new evidence added to concerns first raised by case reports from Finland and Sweden.
A spokesperson said yesterday the new review was expected to be completed by July and would be made public.
The Irish Medicines Board said it was co-operating with the European investigation.
The spokesperson said the Irish watchdog had been notified of four adverse reaction reports involving symptoms of narcolepsy-type sleep disorders suspected in association with the swine flu vaccine Pandemrix.
"Two of these cases are being considered as narcolepsy by the treating doctor while further information is awaited on a third report where narcolepsy is one of several suspected conditions being investigated.
"A further case has been notified by a member of the public for which additional information is awaited."
The swine flu vaccines were produced last winter by GlaxoSmithKline in response to the threat of a pandemic -- 850,000 people received Pandemrix.