The early-morning attack on the south side of the city was the second such attack on an elderly person in their home the city in two weeks.
An elderly woman was terrorised by raiders in her north-side home last week.
Yesterday, two men wearing high visibility jackets and posing as gas company workers knocked on the door of 65-year-old Martin O’Sullivan’s home on Gilabbey Street at around 8.20am.
When he opened the door, they kicked it in and forced their way into his home before stuffing a sock in his mouth.
It was the seventh time Mr O’Sullivan’s home has been targeted by raiders in recent years but the first time he was attacked.
Mr O’Sullivan was pinned down on a bed as the raiders demanded money. But alert neighbours heard his screams for help.
Next-door neighbour Seán Hartigan rushed to Mr O’Sullivan’s door as his partner, Louise O’Donovan, phoned the gardaí.
Mr Hartigan, a 6ft tall weightlifter, shouted that gardaí were on their way and waited outside.
He tackled one of the raiders as they fled and held him until gardaí arrived.
That raider, who is in his 30s and from Carrigtwohill, was arrested under Section four of the Criminal Justice Act and taken to the Bridewell Garda station for questioning. He is known to gardaí.
Gardaí were last night still trying to trace the second raider.
He is described as five foot nine and in his 30s, and was wearing beige trousers, a baseball cap and a dark coloured top. He was wearing glasses as part of his disguise.
Mr Hartigan described the attack on a defenceless elderly man as “an absolute disgrace” and played down his role.
“How anyone could rob an elderly man like this is just beyond me,” he said.
It is understood that up to €50 was taken in the robbery.
Mr O’Sullivan suffered injuries to his head and was treated for shock.
Garda Superintendent Michael Finn issued a warning to elderly people to be wary of callers to their door.
“People should call the gardaí if they have any suspicion at all about who is at their door,” he said.
“But what we have learned from this case is that the best prevention is an alert neighbour.”