The pair, in their 20s, are currently in the intensive care unit of a Drogheda hospital.
One of the women fell ill on Monday night and remains on life support machine. She has not regained consciousness since being admitted to hospital.
Less than 48 hours later, the second woman became unwell.
It is believed both women used cannabis within days of feeling ill.
Garda say the drugs pose a "serious public health concern".
The reaction is not normally associated with cannabis, and Irish police and the Health and Safety Executive are working to establish if a 'bad batch' is being sold in the area.
Gardaí raided two houses in Co Louth, one of which is in Drogheda. Drugs seized during the operation are being tested to determine if they are linked to the investigation.
Garda Superintendent Gerry Smith of Drogheda Garda Station said: "We are concerned that other people who use cannabis may suffer the same effects."
Anyone who has used cannabis in the Co Louth area has been urged to contact their doctor.