Seven suspects, including two women, are being questioned by gardai investigating allegations that a young girl was raped by up to a dozen men over a five-year period.
Those detained include the girl's mother and brother.
The offences are alleged to have taken place when the girl was aged between four and nine at a number of houses, mainly in south county Dublin. The girl is now 10.
The authorities became aware of the alleged sexual activity involving the girl last August. She was put into care last year on the basis that she had been neglected by her mother and while living in a foster home told how she had been sexually abused.
The HSE and gardai were alerted and the girl gave 14 hours of interviews to specialist child interviewers.
It is alleged that she was taken to houses in Dublin by her mother and then abused by a number of men.
An investigation was then launched into the allegations and efforts made to track down those said to have been involved in the long-running abuse.
Gardai said the incidents were alleged to have taken place between 2007 and 2012, with most of the incidents occurring between 2008 and 2011.
A senior officer said last night: "These are horrific allegations and have resulted in widespread inquiries over the past six months to determine what took place and who was involved."
The investigation culminated in a major operation yesterday in counties Wexford and Carlow as well as Dublin's south inner city and Crumlin and Tallaght on the southside of the capital.
Gardai believe between 10 and 12 men were involved in the abuse but some of them have since left the country.
Detectives have been in contact with police forces in other jurisdictions as part of the investigation.
Officers from Sundrive Road garda station, backed up by members of the domestic violence and sexual assault unit within the national bureau of criminal investigations, spearheaded the operation.
Five men and two women, including the girl's mother and brother, were detained in the raids. Six of them are eastern European and the seventh is a British national. They were being held for questioning at Sundrive Road, Kilmainham, Terenure, Rathmines, Tallaght and Rathfarnham stations.