I'm from Ireland and I think the strangest one here is the disappearance of Trevor Deely in 2000. He attended a work Christmas party, was seen on CCTV entering his work place at around 3:30am. He was captured leaving his office at around 4am and then for the last time at 4:15am on a CCTV camera near an AIB bank on the corner of Baggot street bridge. He was never seen again and almost 24 years on, no trace of him has ever been found. A hooded figure who was spotted on camera outside his workplace has never been identified and considered by many to be a possible suspect. The Peter Bergmann John Doe case in Sligo is also a bit unusual. Unidentified man who went by the alias of Peter Bergmann arrived at the Sligo city hotel in June 2009. He was seen behaving strangely over several days, leaving the hotel multiple times with a purple bag filled with unknown items. Several days after arriving in Sligo, his partially dressed body was found on the beach at Rosses point. An autopsy found that he was terminally ill with two different types of cancer and heart disease. Despite extensive investigation, his true identity, motives and even how he arrived in Ireland have never been solved.

https://www.irishtimes.com/ireland/2023/12/09/man-walking-behind-trevor-deely-on-night-of-disappearance-had-already-made-garda-statement/