I can see exactly how it happened.
He was reclusive, had spent a lot of his life living in the UK, having moved there as a small child with his parents (all his younger siblings were UK-born, and they only came to Kerry on holiday), and seems to have been in touch with his siblings (who were scattered across Ireland, the UK and Australia) only in a sporadic way (the sister who spoke at the inquest didn’t know exactly when he had married and moved back to Ireland with his English wife, only that the marriage had been very short, and that his wife had left again for the UK).
The reason the Australia-based sister came looking in Mallow was because she didn’t get a Christmas card in 2002. That was apparently their only contact. Local people assumed he’d gone back to the UK. He clearly didn’t engage at all with other people, even his own family. He didn’t have friends. People didn’t see him. When the house was boarded up, no one thought he might still be inside. That’s the most mysterious bit, that his siblings didn’t break in, or report him missing, but I can see why, too.
It’s a horrible story. But far from difficult to believe.
My own uncle could easily have died in this way — we still don’t know what happened to him for almost 30 years, after he stopped writing home after he moved to Manchester from rural Ireland in the 50s. His brother went over looking for him several times, traced former employers and addresses, reported him missing etc. but hard to trace someone working casual construction jobs and living in poverty in boarding houses. A priest who came across him in a homeless hostel finally made contact with his family in the 80s, and he was brought home a broken man with significant MH problems. He never spoke about the missing years.