A 22-year-old suspect was arrested late on Wednesday night. He is the fifth person arrested in connection with the investigation, which was upgraded to murder within days of her disappearance.
She is believed to have been killed after attending a party at a caravan site in Ballyhalbert, on the Ards Peninsula in Co Down.
Individuals linked to loyalist paramilitaries are suspected of being involved in the 25-year-old’s disappearance. But no trace has been found of the body, despite air, land and sea searches.
Her family offered £10,000 for information to the capture and conviction of her killers.
British Prime Minister Tony Blair and the Government have both pledged their support in helping to find Ms Dorrian’s body.
A forensic expert brought in to help trace the so-called Disappeared - IRA victims murdered and secretly buried at the height of the violence in Northern Ireland - is also expected to examine the case.
Detectives have already questioned and released four men about the killing.