Just over a third of the workforce will be covered by a new law forcing employers to reveal the pay gap between their male and female staff when it is first rolled out.
A Government bill to be published soon will initially apply to businesses with more than 250 workers.
There are 466,774 people working in these firms, or 34.9pc of the total private sector workforce, according to figures supplied by the Department of Justice and Equality.
But it will be up to three years after this before another 290,275 workers in firms employing 50 to 249 staff - a further 22pc of the workforce - are included.
Workers at firms where there are fewer than 50 staff will not be included at all.
Sources said the first deadline for the big firms to submit the mandatory pay gap surveys is likely to be December next year or January 2021.
But the Irish Congress of Trade Unions and the National Women's Council want the Government to ensure all employers publish the data within a year of the legislation coming into force.
This is justified because it "has long been signposted", they said.
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