Irish Ghost Hunters chief Tim Kelly said paranormal activity is particularly strong in the three ancient castles which make up the county's famous “haunted triangle,” namely, Kinnitty, Leap, and Charleville.
The Dublin-based spook detector said: "There's no doubt in my mind that County Offaly is the most haunted county in Ireland, indeed probably one of the most haunted areas in Europe.
"Over the years we've conducted hundreds and hundreds of paranormal investigations and there's something about Offaly, particularly the energy around that haunted triangle."
Recalling an overnight poltergeist probe in Kinnitty Castle - a popular four-star hotel - a number of years ago, the 49-year-old said: "It was a pretty spooky experience. The eeriest part of it was when I was leaving one of the big old rooms there and I suddenly heard my name, 'Tim' being whispered.
Kelly - whose six-man team use hi-tech gadgetry, including thermal imaging cameras and powerful audio equipment for their investigations - said he's encountered similarly creepy experiences at nearby Leap and Charleville.
He said: "Over the years we've probably done over 100 investigations at Wicklow Gaol, and probably every second or third visit we'd pick up something, which is not surprising given all the cruelty, suffering and death that took place in the jail.
"I don't like the atmosphere in the Hellfire Club. We've done about five or six investigations there, and it's not somewhere I particularly want to return to. There's a very cold and negative vibe there."
Meanwhile, Kelly, who when not ghoul-busting works as a radio presenter, and his crack team said they hope to conduct their first-ever paranormal investigation at Dublin's Shelbourne Hotel.
One of the rooms in the luxury five-star retreat is said to be haunted by the ghost of Mary Masters, a mischievous seven-year-old girl who died of cholera in 1791.
Where's the spookiest place you've ever been in Ireland? Let us know in the comments section below.
*Originally published in October 2017, updated in October 2022.