The RTÉ presenter was 53. The cause of death was not disclosed. Ryan had been living alone since he separated in 2008 from his wife Morah, with whom he had five children aged 10 to 24.
According to the Irish Times, an investigation is being conducted, but foul play is not suspected.
RTÉ Radio's managing director Claire Duignan told the Irish Times that Ryan had complained of not feeling well last night and told colleagues he didn't think he would be able to do his show today. "When he spoke with people to say he wouldn't be in today, they obviously expressed concern and asked did he need a doctor or anything like that," she said. "But he said no, he was fine, but he wouldn't be in today."
Ryan's body was found at his home at 1 p.m., and then the media flocked outside his basement-apartment. A doctor arrived at about 3 p.m. and left within 20 minutes. An undertakers' ambulance arrived shortly after 3:45 p.m. At 4:15 p.m., Ryan's body was removed through the basement doorway and placed in the ambulance, which left the scene accompanied by an unmarked Garda car, the Irish Time reports.
Since 1988 Ryan, a Dublin pioneer of pirate radio, had hosted a morning chat-and-music show on national broadcasters RTE. He also fronted numerous RTE television talk shows and celebrity interviews and was known for his irreverent, coarse humor.
In a brief statement issued on their behalf by RTÉ, Ryan's family said they were in shock, the Irish Time reports. "Gerry Ryan died today," it said. "Morah and his children are in complete shock. Please respect their privacy at this time."
RTÉ also said it learned of Ryan's sudden death "with profound sadness...the thoughts and prayers of all RTÉ staff are with Gerry Ryan's family and his friends."
Prime Minister Brian Cowen led tributes from all corners of Ireland's political and entertainment communities, lauding Ryan as "one of the greats of modern Irish broadcasting."