The HSE says industrial action will have a 'very disabling impact' on hospitals throughout the country today.
Over 2,100 medical scientists are stopping routine lab services for 12 hours - ending at 8 o'clock tonight.
Their pay dispute has led to inpatient and day-case procedures being cancelled.
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Liam Woods, the HSE's director of acute operations, says there'll also be significant disruption to emergency care.