Ambulance workers from the Psychiatric Nurses Association say they have been left with no choice but to go on strike.
Five hundred of them are walking out for 24 hours across the country as part of a dispute over union recognition.
The row has already led to one 24-hour strike, and at 2pm this afternoon, 500 members of the PNA began their second.
They want to be represented by the union of their choice, but the HSE has said recognising any more would not be in the best interests of workers or the public.
Brendan Flynn, a paramedic supervisor, hopes the strike does not cause too much disruption.
Mr Flynn said: "I would hope that it wouldn't overly impact on patients, but unfortunately we've been forced to take this action.
The workers will still deal with emergencies and the HSE says all calls are being responded to.
Some army paramedic crews have been drafted in and patients are being told to still ring 999 or 112 if they need help.
Tony Gregg from the PNA says more strikes cannot be ruled out.
Mr Gregg said: "We intend to continue this campaign for as long as is required and we are simply asking for the HSE to sit down at the other side of the table."
The industrial action continues until 2pm tomorrow.