Dress for Success Dublin has said that women effectively "work for free" for about seven weeks of each year.
The charity said that men are paid an average of 14% more than women for doing the same job.
The charity is launching a campaign to highlight the issue of the gender pay gap.
Founder Sonya Lennon outlines some of the reasons why women are on lower pay rates than men.
"Some of them are external and some of them are internal," she said.
"So we have an issue around women taking up a lot of the lower-paid roles in caring, in childcare, and they are predominantly staffed by women.
"We're also looking in the corporate world at a corporate culture which has become used to paying men at a higher rate than women,
"And also I think for women, particularly when they come back to work after having children."