More than 300,000 workers, including nurses, doctors, teachers and civil servants, are on strike across the Republic of Ireland.
It comes amid an ongoing pay dispute between public sector workers and the Irish government, with a further strike planned on 21 October.
It is expected schools could be closed with hospital appointments cancelled and picket lines formed across the country during the 24-hour strike.
At the end of September, more than 100,000 workers, including nurses, school staff and civil servants, moved to work-to-rule measures .
At the start of October, primary and secondary school teachers in the Republic began work-to-rule action .
Previously almost 97% of Fórsa members, who voted in an industrial ballot, did so in favour of action. It is the Republic's biggest public service union.
Brendan O'Brien divisional organiser of SIPTU, which represents public sector health and administration workers, previously said the "goodwill" of workers was a large part of what kept public services going.
However, he said the Irish government's failure to establish a basis for formal talks "left our members with no alternative but to act collectively".
On Tuesday Jack Chambers, the Republic's minister for expenditure, described the strike action as "deeply disappointing" saying it would cause significant disruption to the economy, with thousands of hospital appointments set to be cancelled and schools closed.
