Halpin will take up her new position on February 2, replacing O'Riordan who stepped down as editor in October after more than seven years in the job.
Prior to joining IPC, Halpin was editor of Red and she has also been deputy editor of both New Woman and More!.
IPC Southbank said that it will announce a new editor for In Style in due course.
Jackie Newcombe, IPC Southbank managing director, said: "Our search for this prestigious role spanned three continents but it was very clear that, with Trish, we had the perfect editor already within Southbank.
"The role really is one of the most coveted jobs in publishing and to find the best candidate within the company illustrates the incredible depth of talent we have."
Halpin said: "As a life-long Marie Claire fan I am honoured and excited to be taking the helm of this ground-breaking title.
"I look forward to working with the team on taking forward the brand's fantastic combination of award-winning journalism and inspiring fashion and beauty."
Halpin has edited In Style since September 2006. In the latest round of ABC magazine sales figures covering January -- June this year, In Style delivered a circulation of 180,879, up 1.2% year-on-year.
Marie Claire reported sales of 316,765, down 4.8% year on year.
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