GardaÍ believe two women died and two men fell ill, one of them critically, after taking fake Xanax tablets.
A major investigation has been launched into the tragic events at the old Clonamore House Hotel in Tullamore, Co Offaly, which is now used for homeless accommodation.
The two women who died were named locally last night as Angela Kelly (44), who was also known as Andrea and died in Clonamore House on Sunday, and Amy Mannix (19), who died at the hostel yesterday morning.
Both women had previously lived in Portlaoise but it is understood Ms Mannix is originally from Co Kerry.
A 23-year-old man, named as Aaron Ging, was in a critical condition last night at the Midland Regional Hospital in Tullamore.
A 32-year-old man, named locally as John O'Brien, was also being treated in the same hospital but his condition was not described as life-threatening.
Both men had previously lived in the Portlaoise area.
Officers are also investigating whether the sudden death of another man in Tullamore last Wednesday is linked to the same lethal batch of fake drugs.
A senior source told the Irish Independent last night that detectives believe all five people consumed what are known as 'Xanax sticks' which were purchased on the internet last week.
The criminal who is suspected of sourcing the deadly drugs was previously quizzed about the murder of William Delaney last year - a 56-year-old whose body has never been found despite major searches at the Rock of Dunamase in Portlaoise last June.
Last year the coroner for Northern Ireland described the rising number of deaths linked to counterfeit versions of the anti-anxiety drug Xanax as "an escalating crisis".
Gardaí yesterday announced details of the grim tragedy that has unfolded in Co Offaly.
"On Sunday, gardaí were called to a hostel in Tullamore town, where a woman aged 44 years was discovered in an unresponsive state. She was pronounced dead at the scene a short time later.
"This morning, at the same location, emergency services were called to treat a woman aged in her late teens.
"She was pronounced dead at the scene a short time later and her body has since been removed to the mortuary at Tullamore Hospital where a post-mortem examination will be carried out.
"While at the hostel, a 23-year-old man became unresponsive and was removed by ambulance to Tullamore Hospital."
A spokesperson for Pfizer, the manufacturer of Xanax, previously spoke about the dangers of fake versions of the drug and said they are "alarmed by the rise of counterfeit Xanax" on the internet and dark web, and they estimate that 96pc of the Xanax samples that they analyse in their counterfeit labs are fake.
"We have found dangerous ingredients such as boric acid, heavy metals and floor polish in counterfeit medicines," the company said in 2018.
Last night, a senior source said that a chronic shortage of illegal drugs in Ireland because of the coronavirus pandemic is leading to an increase in people attempting to buy drugs on the internet.
"As you can see, the consequences of this can be very grave indeed."