It has emerged that the sports writer's daughter discovered the image of a 16-year-old on her father's phone.
The teen girl in the picture has since given a statement to gardai in which she admitted some form of consensual sexual relationship took place.
The reporter, who works for a national newspaper, is understood to be on "suicide watch" at a medical facility in Dublin.
Detectives are focused on trying to establish exactly when the affair began as this will determine the severity of charges that the reporter could face.
It is understood that girl is 16 years old but has been known to the man for around two years. Officers have not yet been able to quiz the journalist because of his medical state.
There is no longer a conviction of statutory rape under Irish law but if officers find that the relationship began while the girl was under 15 then the man could face life in prison.
If it is found that inappropriate behaviour began after she was 15, he could face up to 10 years in jail.
Officers are reviewing all phone contacts between the accused and the girl to establish when the affair started.
They are also expected to make contact with several other young girls who have been in contact with the man at a Dublin GAA club -- although there is only a single allegation.
The investigation has been ongoing for three weeks after gardai were alerted to the picture by the man's wife.
The journalist's own daughter -- also a teenager -- was given the phone to dispose of as part of a recycling drive. She put her own Sim card into the mobile and found a naked photograph of a girl who is known to her.
The Herald understands that the accused's daughter is a member of the same club as the girl involved. She told her mother about the image and gardai were then alerted.
Officers from the Garda Domestic Violence and Sexual Assault Unit in Harcourt Square immediately launched an investigation.