Senior officers were in contact with the DPP throughout the night as detectives continued to question the 24-year-old local man.
His detention period was due to end overnight at which stage the man, who knew Ms Phelan, either had to be charged or released again.
The man was first arrested last Monday, but officers were forced to release him on Tuesday night after the DPP ruled against a charge.
Detectives re-arrested him on Wednesday after Ms Phelan’s body was found, buried several feet under a field adjoining a house on the Timahoe road, 5km outside of Portlaoise.
Ms Phelan’s family, from Ballyroan, about 11km from Timahoe, had to go through the trauma of identifying the 30-year-old’s body after an autopsy was completed late on Wednesday.
The autopsy results have not been made public for what gardaí say are operational reasons.
Ms Phelan, a nanny who was four months pregnant and was described by one garda as a “slip of a thing”, went missing in Portlaoise on Oct 25.
The investigation has proceeded at a frantic pace since gardaí upgraded what was a missing person’s investigation to that of murder last Monday. That afternoon, gardaí began searching the house on the Timahoe road and arrested the 24-year-old man.
The search at the house and adjoining shed and land continued until the focus dramatically shifted to a stretch of the River Barrow, 20km away, near Monasterevin on the Laois-Kildare border.
Gardaí had received specific information that Ms Phelan was dumped over a certain bridge into the river. A search, involving underwater Garda divers, was put into operation.
Around midnight on Tuesday the 24-year-old was released without charge. On Wednesday morning, the focus shifted back to the house and to an adjoining field at the rear of the property. A mini-digger was used and shortly before noon gardaí spotted human remains buried deep in the field. That day, just before 7pm, the man was re-arrested and taken to Portlaoise Garda Station.
Floral tributes and teddy bears were placed yesterday at a gate leading to the property where Ms Phelan’s body was recovered.
Meanwhile, a tribute page on Facebook was taken down following a request from her family. The page was set up after the body was discovered.
By yesterday morning the site had attracted more than 4,500 likes with many people leaving messages of sympathy.
Aoife’s sister Leighanne Roche posted on the page: “Whoever started this page please take it down in respect of my sister Aoife. Thanks.”
Last night, people attended a memorial mass for Ms Phelan, held at St Patrick’s Church, Ballyroan, Co Laois.
Ms Phelan is to be laid to rest following funeral Mass at 2pm tomorrow at the church.