The 41-year-old was arrested at his home in leafy Foxrock, south Dublin, where he lives with his wife and two children.
Searches were conducted throughout the morning at the home and gardens. Detectives removed at least one computer; mobile phones; and two vehicles — which will be technically examined.
The suspect is in a senior position in a professional practice, where he has worked for more than 10 years.
Garda sources said he had a “clean bill of health” in terms of criminal convictions and has never come to their attention.
Partial remains of Ms O’Hara, aged 36, were discovered in the foothills of the Dublin mountains last Sep, just a year after she was reported missing.
Then it emerged that belongings linked to Ms O’Hara, including a Dunnes Stores loyalty card, were found in a reservoir in Roundwood, Wicklow, 40km away by road.
DNA tests on dental remains at the scene confirmed it was Ms O’Hara, while the loyalty card suggested she had previously been at Roundwood.
Handcuffs, ankle-cuffs, and adult items also found at the Roundwood site were linked with the deceased.
Examinations of her laptop showed she had visited a number of adult sites and had been in contact with a number of people.
Detectives examined her mobile phone found at Roundwood and traced phone numbers that had been in contact with her. They studied who she had conversed with on the sites and identified them. This “intensive” investigation led to yesterday’s arrest.
The suspect was taken to Blackrock Garda Station and detained on suspicion of Ms O’Hara’s murder, under section 4 of the Criminal Justice Act.
Ms O’Hara was last seen leaving her home Belarmine Plaza, Stepaside, at around 5pm on Aug 22, 2012.
A reported sighting of the woman placed her close to the footbridge at Shanganagh Park, Shankill, at around 6.15pm on the same day.
Her car, a turquoise 05D Fiat Punto was located at Shanganagh Cemetery two days later.
Gardaí suspect the childcare assistant may have arranged to meet someone at Shanganagh before travelling to Roundwood, where she was killed.
Detectives believe the items were dumped into the reservoir and that her body was taken to the Killakee mountains. Officers said Ms O’Hara, weighing 16 stone, would not have been easy to move.
Gardaí have been hampered by the fact that haven’t been able to establish a cause of death for Ms O’Hara. Searches at Killakee mountains have not been able to locate her skull or a key neck bone which might indicate is she was strangled.
Remains that were found did not reveal any signs of violence. Gardaí said it was possible an animal may have taken her skull.