He was giving evidence in the trial of a 35-year-old Wicklow man charged with murdering a young father at his Tallaght home in March 2009.
Garrett O'Brien, of Clover Hill, Bray, is on trial accused of shooting dead father-of-two Seamus O'Byrne in front of his girlfriend and baby son.
A man wearing a dark, hooded tracksuit shot the 27-year-old five times in the driveway of his house at Tymon Park North on March 13.
Detective Garda McGrath was being cross-examined by the defence yesterday on his identification of a man in CCTV footage.
The detective had already testified that he believed O'Brien was the man seen in various video clips making purchases in shops in Bray and Dublin in the days before the killing.
He agreed with Feargal Kavanagh, defending, that he had no qualification in facial recognition.
"I'm well used to looking at CCTV and CCTV stills and recognising people. Either you recognise the person or you don't," he said. "In my opinion, you don't need a facial recognition expert to recognise people ... It's not a science."
He was asked if he was aware of 'photoshopping' and airbrushing and he said he was.
"None of it was 'photoshopped' or airbrushed in any way," he said. "None of the CCTV was changed in any manner."
He agreed he did not know the accused before he was arrested four days after the killing, when he saw him for a few minutes. He also did not know what colour eyes he had.
However, he said he was 'absolutely not' trying to fit O'Brien into the CCTV images.
"I recognise Mr O'Brien on five occasions in the CCTV," he said, adding that he recognised him from his facial features, stature, demeanour and scar on the left side of his head.
Mr Kavanagh suggested that lots of people can have scars in the same place.
The detective said he was working off common sense.
Mr Kavanagh asked him to again look at the footage of a man walking into a shop in Bray.
"That's Garrett O'Brien, Judge. I can't bring it any further," he said. "It's common sense."
He was asked about his description of the man's gait as 'unusual'.
"Well, I don't see too many people walking that way," he said. "If you look at that clip of Garrett O'Brien walking around the Spar, you can see his feet pointing outwards."
Mr Kavanagh asked if there would be anything sinister or unusual about his client going into a shop in Bray and making the purchases made. "No. He's from Bray," replied the detective.
The trial continues.