O'Riordan, who had overseen a substantial relaunch of the magazine in September 2006, leaves following a 4.8% year-on-year decline in circulation to 316,765, according to the latest ABC figures.
Marie Claire, which positions itself as magazine promoting 'fashion with heart', features long reportage pieces alongside more traditional real-life, celebrity and fashion features.
O'Riordan said she was going out on a high, adding: 'After a long and happy stint on Marie Claire, I'm off to fresh woods. It's been a privilege to work for a magazine whose readers - I think uniquely - manage to combine seriousness with fabulousness and a little kindness.'
O'Riordan, a former editor of Elle, took over at Marie Claire from Liz Jones in 2001, when the magazine's circulation was about 400,000. Jones has since forged a career as an author, and columnist at the Daily Mail and Mail On Sunday.
Marie Claire is a joint venture between IPC and the French Marie Claire Group.
IPC says a successor for O'Riordan, who will leave in December, will be announced in due course.
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