The HSE move to stop treating stroke patients at Our Lady's Hospital in Navan, Co Meath, has been described as "very concerning".
An emergency meeting of the Save Navan Hospital Campaign is due to be called to "mobilise people" on this matter.
The bypass of patients came into effect at 8am yesterday, according to a directive sent to ambulance and call centre staff.
"Patients with signs and symptoms of stroke (including transient ischaemic attacks) should not be brought to Our Lady's Hospital, Navan," it says.
"Instead, these patients should be transported to the next nearest hospital emergency department that provides stroke thrombolysis."
These hospitals are listed as Dublin's Connolly and Mater hospitals, the Regional Hospital in Mullingar, Co Westmeath, Our Lady of Lourdes Hospital in Drogheda, Co Louth, and Cavan General Hospital.
It adds that all "self-presenting patients to Navan with subsequent confirmed stroke" will be brought by ambulance to the Mater Hospital.
The rationale behind the decision was that "stroke patient outcomes are optimised by treatment in centres with higher volumes of stroke treatment".
Meath West Aontu TD and chairman of the Save Navan Hospital Campaign Peadar Toibin expressed his concern.
"This is a significant downgrading of the hospital and comes on the back of significant disinvestment of services," he said.
"I'd be very concerned for patients of stroke in Meath who need to get to hospital as fast as they can.
"Time is of the essence in these cases and often, delays in the response times of ambulances can be lengthy as it is, without more time being added to get to a hospital further away.
"It's another foundation taken away from the emergency department.
"Government policy wants to close it and there have been many threats in the past.
"If an ambulance has to travel to Dublin and wait with a patient, it is out of action for hours, which means it will lead to even lengthier ambulance response times.
"As chair of the Save Navan Hospital Campaign, I'll be forced to call an emergency meeting to mobilise people on this issue."
The move was met with "disappointment" by stroke victim and radio DJ Gerry Stevens, who is currently broadcasting a series of podcasts with stroke victims and medics.
"It's not good. Time is brain, and the more minutes you lose, the more damage you do to your brain," he said.
"I was three minutes without oxygen. Two more minutes would've left me needing treatment in a care home due to the level of damage to the brain."
Gerry (52), from Duleek, Co Meath, suffered a stroke in November 2017 and spent three months in Our Lady of Lourdes Hospital in Drogheda - the closest hospital to his home.
"Minutes are critical," he said.
"It's not the direction you want to go in at a time when 2,000 of the 10,000 who suffer stroke each year in Ireland die.
"One in five will suffer a stroke at some stage.
"The quality of life and how much of you that comes out the other side depends on time, and we need more investment, not less, in local services."