Text messages on a mobile phone dumped in a reservoir led gardai to arrest the businessman suspected of killing childcare worker Elaine O'Hara (36).
The man is due to be released from garda custody shortly.
It's understood he was questioned throughout the night by gardai, after he decided to forgo a break from questioning.
The man's period of detention is due to expire soon.
Gardai have been liaising with the Director of Public Prosecutions to on whether they should charge him, or release him and send a file.
The compnay director - a married father of two from Foxrock in South Dublin - was arrested after being released from hospital.
He was identified within a week of the decomposed body of the 36-year-old (pictured) being found, and officers spent several more weeks trawling through data retrieved from a mobile phone and a computer belonging to the victim.
A decision will be made today on the fate of the 41-year-old professional, with two children, who was detained at his home in the south county Dublin suburb of Foxrock at breakfast time yesterday.
The arrest was carried out by officers investigating the murder of Ms O'Hara, whose body was found in the Dublin mountains more than a year after she had disappeared.
Gardai had been carefully eliminating possible suspects from their inquiries after their names emerged in a check on a website being used by Ms O'Hara to meet men with common interests.
The man who was arrested yesterday had been been getting frequent medical attention in the aftermath of the discovery of Elaine's body.
Over the past two weeks, officers had become increasingly hopeful that their progress in the investigation would lead to an arrest.
The breakthrough came shortly after 7am yesterday when officers swooped on the professional's home, where he lives with his wife and two children.
Gardai seized phones and computers from the house, as well as two vehicles for forensic examination. This followed a detailed search of the suspect's home and surrounding grounds.
Officers also visited the man's offices, where he holds a senior position, in the city centre.
The suspect was whisked away to Blackrock garda station, where he was questioned throughout the day and visited by his lawyers.
He was detained under Section 4 of the Criminal Justice Act, which allows gardai to hold him without charge for up to 24 hours, excluding rest periods.
As well as the mobile checks, gardai also pored over 1,600 hours of footage taken from CCTV cameras in a series of locations, including those close to the victim's home at the Belarmine plaza in Stepaside, Co Dublin.
Gardai think Ms O'Hara met her killer within a few hours of leaving her home on August 22, 2012. They also believe she had been in his company previously.
They are working on the basis that she struck up a friendship with the man on an internet website as a result of their shared interests. It's understood it was a website on which adults interested in bondage meet.
An examination of her computer shortly after she was reported missing by her father established that she was an active internet user.
It quickly became clear, after her body was found last September, that this would be a key line of inquiry.
The remains of the 36-year-old were discovered in undergrowth, not far from the Killakee road, outside Rathfarnham, in the foothills of the Dublin mountains. The find was made by a woman who went into the undergrowth to look for her dog.
Gardai were alerted and during an intensive search they discovered a number of bones and a set of teeth, which allowed forensic officers to positively identify the remains as being those of Ms O'Hara.
By coincidence, two days earlier, a caretaker at a reservoir in Roundwood, Co Wicklow, had spotted a number of items that had emerged because of low water levels.
Members of the garda water unit were called in and during searches Ms O'Hara's mobile phone was recovered, along with a key fob – which identified her as a customer of Dunnes Stores – some of her clothing and other items, including handcuffs and restraints.
Ms O'Hara, who was a childcare worker and also had a part-time job in a newsagent's shop in Blackrock, had been seen leaving her home at 5.05pm on August 22 and at around 6.15pm she was reportedly spotted near a footbridge over the rail line at Shanganagh in Shankill.
Gardai believe that she visited her mother's grave in Shanganagh cemetery.
Two days later her turquoise Fiat Punto was located by gardai at the cemetery.
Ms O'Hara was described by gardai as a trusted person and a very popular employee.
On the day she went missing, she had volunteered to work at the Tall Ships festival in Dublin city centre.