On the evening of the murder, Gardai questioned O'Reilly in his mother's house in Dunleer. He told Gardai he slept in a separate bedroom to his wife the night before the murder because he had to get up early and he didn't want to "disturb" her.
He told them he woke up at 5.20am and went to Jackie Skelly's gym where he met his co-worker, Derek Quearney. Mr Quearney, from Balbriggan, later became his alibi. In the car park, the two discussed their work plans for the day as they waited for the gym to open.
O'Reilly told Gardai they only had a shower and a sauna as they had a busy day ahead.
O'Reilly then went to work in Viacom, the outdoor advertising company where he worked as operations manager in Bluebell Industrial estate. CCTV footage showed him entering and leaving the building.
At 8.25am he left under the guise of inspecting posters in the Broadstone bus garage in Phibsboro, where he said he met Mr Quearney. In reality, he drove to his house in the Naul, waited for his wife to return from dropping their children to school.
When Gardai asked O'Reilly about his relationship with his wife, he admitted they had been having difficulties. Initially he denied having an affair but then told them about Nikki Pelley, his lover. Det Sgt Patrick Marry said O'Reilly told them: "the affair was over and he didn't want his family to know."
He also told Gardai that one of the reasons he and Rachel were not getting on was because she wanted to have another child because she wanted a daughter. He told Gardai that he didn't know why she wanted this because they already had the two boys whom he loved very much.
During the course of this first witness interview, O'Reilly "pointed the finger of suspicion" at a number of people. He told Gardai about several Viacom employees whom he'd fired, neighbours with whom he claimed Rachel had been having a dispute over land, and Rachel's natural family, with whom he said Rachel's relationship "wasn't too hot."
In the weeks following her daughter's death, Rachel's mother, Rose Callaly, said O'Reilly was a 'regular visitor' in their house. On one occasion, he told them they would hear rumours he was having an affair but that it wasn't true. She said he also told them there would be second rumour alleging she and her husband had abused the deceased.
Two weeks after the murder, on 22nd October, O'Reilly appeared on the Late Late Show with Rachel's mother Rose Callaly, making an appeal for information about the murder. The Callalys gave evidence that he separated from them after the TV show. In her evidence, Ms Pelley said he spent the night with her.
Mr O'Reilly was arrested twice on suspicion of murder. Once on November 17th 2004, and a second time on March 4th 2005.. In these interviews, Mr O'Reilly repeatedly denied anything to do with his wife's murder. Asked whether he regretted murdering his wife, he said: "I have regrets my wife was murdered."
However Gardai had formed a suspicion of his guilt and they began to probe into the account he had given of his movements. They took possession of his car and drove it past the Murphy's Quarry CCTV camera to create comparative CCTV footage.
They then traced and timed the journey they believed he took on the morning of the murder.
But the most definite evidence came from the analysis of traffic on mobile phone masts. Expert analysis of mobile phone records showed a "different picture" to the one created by Mr O'Reilly.
This analysis showed a mobile phone mast at Murphy's Quarry near the O'Reilly household had picked up a signal from Mr O'Reilly's phone twice on the morning of the murder.
At 9:25am, Mr Quearney called Mr O'Reilly. In his evidence, Mr Quearney said he did this to see where Mr O'Reilly was and that Mr O'Reilly told him he was at the back of the pits in the garage. The call lasted for just over 2 minutes. This call was picked up by the mast in Murphy's Quarry.
At 9.52am, a friend of Mr O'Reilly's called Ciaran Gallagher sent a text to his phone telling him he couldn't meet Mr O'Reilly for lunch as planned.
This was the second communication picked up by the Murphy's Quarry mast.
Experts gave evidence that it would be "impossible" for the mobile phone mast in Murphy's Quarry to pick up the signal from Mr O'Reilly's phone in the Broadstone Phibsoboro garage.
One of them, Eddie Gleeson, said it would be "extremely unlikely," because the network is designed so as not to let this happen. "It would cause huge engineering problems for us," he explained. He also told the court that on the day of the killing, calls made from Mr O'Reilly's number were from the same handset and same SIM card at all times that day.
Among the many calls to and from Mr O'Reilly's phone that day, there was a total of 18 communications between Mr O'Reilly and his lover, Nikki Pelley. Three calls between their phones in the hours before the murder lasted a total of 58 minutes and 25 seconds.
The CCTV footage and mobile phone analysis came together to incriminate Mr O'Reilly at 10.07am. At this time, an image of his car at Blake's Cross, on the old Dublin / Belfast road, on the return journey from the murder scene, coincided with the time Mr O'Reilly sent a text to his wife's phone, asking her how she was.
This text was picked up by a mast very close to Blake's Cross, meaning they had both CCTV footage and mobile phone masts of Mr O'Reilly at the same place and time. Gardai spent "weeks" looking at 119 separate pieces of CCTV footage.
Mr O'Reilly wasn't seen in any of the footage from the depot and no one who worked there saw him either. The only time he was captured on footage was when his car was seen leaving at around 11am with Mr Quearney.