The son of property developer Gerry Gannon was found dead in his living room having taken methadone, an inquest heard.
Peter Gannon (31) was found lying on the couch by his uncle at the home they shared at Park Avenue in Clongriffin, Dublin 13, late on July 6 last year.
Dublin Coroner's Court heard that Mr Gannon had methadone in his system when he died, as well as therapeutic levels of prescribed anti-depressants.
The post-mortem also revealed that he was suffering from undiagnosed severe heart disease, which coroner Dr Brian Farrell said may have been a contributing factor in his death.
He was found by his uncle Stephen McDermott who told the coroner that he had been trying to contact his nephew since the previous evening with no success.
When he left the house to go down the country at 7.30pm the previous night, Mr Gannon was having dinner and was "in great form" and "joking", he said.
When he arrived back at the house, the lights were off and he thought his nephew was in bed, he told the inquest. He went into the house and there was a "staleness in the air", he said, before he went to the living room and saw his nephew on the couch.
"I knew there was something wrong with him. I asked him to wake up," he said.
"I turned on the light and I saw his colour. I knew he was gone. I ran from the house."
Mr Gannon had been dead for several hours when he was found. Dr Farrell said that, at post-mortem, a "significant amount of methadone" was found in Mr Gannon's system as well as therapeutic levels of his prescription medications. The family said that they were "shocked" at the presence of methadone.
Mr McDermott said his nephew had some difficulties with depression and suffered from panic attacks but seemed to be "getting over" it. He was "absolutely not" on the methadone programme, he added.
"He would never ever, not in a million years, be near heroin or methadone," he said.
Dr Farrell told the family that pathologist Dr Muna Sabah also found near complete narrowing of one of the major arteries in Mr Gannon's heart. This could have been a contributory factor, he said.
Speaking from the body of the court, Mr Gannon's aunt Ann Mooney said two weeks before he died, he felt heart palpitations which he thought were panic attacks so he went to a GP and A&E in Sligo General Hospital.
His parents Gerry and Margaret Gannon were present in court along with the rest of their family.
From the body of the court Mrs Gannon said that her son had been seeing a new doctor and was not happy with the medication he was on.
"I am just looking for answers," she said.
Dr Farrell gave the cause of death as acute cardio-respiratory failure due to the toxic effects of methadone and medications with severe coronary artery narrowing and heart enlargement.
He said he was satisfied there were no suspicious circumstances and no evidence of deliberate self-harm. He returned a verdict of death by misadventure.