Gardai believe a criminal, who was questioned last June about a murder, bought deadly drugs on the 'dark web' which were responsible for the deaths of two women.
Two men were also admitted to hospital in a serious condition.
A major garda investigation is ongoing into the tragic events at the old Clonamore House Hotel in Tullamore, Co Offaly - which is used for homeless accommodation - where it is suspected that four residents of the facility took fake Xanax tablets.
The two women who died were named locally as Angela Kelly (44), who was also known as Andrea and died in Clonamore House on Sunday, as well as Amy Mannix (19), who died at the hostel yesterday morning.
Both women had previously lived in Portlaoise but it is understood that Ms Mannix was originally from Co Kerry.
A 23-year-old man was in a critical condition and fighting for his life last night at the Midland Regional Hospital in Tullamore.
Another 32-year-old man was also being treated in the same hospital but his condition was not described as life-threatening.
Both men had previously lived in Portlaoise.
Officers are also investigating whether the sudden death of another man in Tullamore last Wednesday is linked to the same lethal batch of dodgy drugs.
A senior source told the Herald 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 batch was last year quizzed about the murder of William Delaney, a 56-year-old whose body has never been found despite major searches at the Rock of Dunamese in Portlaoise last June.
In 2019, the coroner for Northern Ireland called the rising number of deaths linked to counterfeit versions of the anti-anxiety drug Xanax "an escalating crisis".
Gardai yesterday announced details of the Co Offaly tragedy.
"Gardai in Tullamore have commenced an investigation following the unexplained deaths of two women in Tullamore, Co Offaly," a spokesman said.
"On Sunday, gardai 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.
"Her body was removed to Tullamore Hospital for a post-mortem examination.
"This morning [Monday] 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 will be carried out.
"While at the hostel a 23-year-old man became unresponsive and was removed by ambulance to Tullamore Hospital.
"His condition is understood to be critical. A second man aged 32 years was also taken to hospital for treatment. His condition is not thought to be life-threatening.
"Gardai are liaising with the HSE and CADS, a local community alcohol and drugs support group, in relation to the case.
"The deaths are not thought to be Covid-related and gardai are awaiting post-mortem and toxicology results to determine the course of a their enquiries," the spokesman added.
A spokesperson from Pfizer, the manufacturer of Xanax, previously spoke about the dangers of fake versions of the drug and said they were "alarmed by the rise of counterfeit Xanax" on the internet.
"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 - this seems to be a very bad batch," the source said.