Some 50 to 70 Nigerian failed asylum-seekers were being detained last night in Dublin in advance of imminent deportation, according to refugee support groups.
Young people and a heavily pregnant woman were among those held at two centres in Dublin, according to the groups.
A specially chartered flight was due to fly out early this morning, they said.
The Garda National Immigration Bureau (GNIB) refused to comment on the issue.
The family of at least one Irish-born child was also due for deportation, according to a spokeswoman for the Children's Rights Alliance.
Failed asylum-seekers were brought to Dublin in buses from many parts of the country yesterday afternoon and were told that they would be deported within 24 to 36 hours, the groups reported.
Mr Peter O'Mahony, chief executive of the Irish Refugee Council, said he was aware of a substantial number of Nigerians being held. This operation had been ongoing for the last six weeks, he said.
Relatively few Nigerians have been deported to date because the Nigerian authorities were unwilling to enforce a repatriation agreement with the Government, he said.
He said the Government should meet its own obligations under the agreement, particularly with regard to deporting young people. A number of young people who arrived in Ireland as unaccompanied minors or separated children were being detained for deportation, he said.
"They are particularly badly equipped to reintegrate themselves into their country."
Ms Rosanna Flynn of Residents Against Racism said one heavily pregnant woman was being detained last night. She criticised the "terrible secrecy" surrounding deportations and called for a transparent system that allowed people to talk to their solicitors.