The 64-year-old man who was arrested for questioning about the disappearance of Mary Boyle has been released by Gardaí without charge.
He returned to the Midlands Prison last night, where he is serving a sentence for assault on young boys.
Mary Boyle disappeared in 1977, aged just six. She had been visiting her grandparent's house in Cashelard, near Ballyshannon in Donegal. Since the initial police investigation at the time, there have been two other Garda reviews, but no trace of Mary has ever been found.
The arrest earlier this week was made after new evidence was found by the Review Team for Mary's case, which is based in Northern Ireland.
Mary's mother Ann last night praised the Gardaí for their ongoing work since Mary's disappearance, which is Ireland's oldest missing persons case. "They have always done their best," she told the Irish Independent.
"Please God we will still find her. I always felt something happened to Mary and her body is out there somewhere. All I ever wanted was to give her a Christian burial and it is my wish that I can still do that, bury her now beside her father, Charlie."
She also pleaded for anyone who might have details about what happened on the day Mary disappeared to speak out. "Just give the information to someone so we can find her; that's what I would say to whoever took her," she said.