He joins the peer-to-peer (p2p) lender from Paragon Bank, where he was head of development finance.
O’Riordan (pictured) previously spent a decade at Nationwide and held senior lending roles at Bardford & Bingley, and Barclays.
During his time at Paragon, O’Riordan was responsible for driving growth and helped enhance the firm’s reach in the Midlands and the North.
The appointment is the latest in a series of new hires at Relendex.
Commercial director Paul Sonabend recently joined the lender, as well as new business development managers and a new underwriter.
Relendex is aiming to lend £100m in new loans in 2019.
Michael Lynn, chief executive at Relendex, said: “At Relendex, we are always looking for ways to take the company into the next stage of its growth.
“Fintan’s appointment is a clear demonstration of this.
“Fintan has an impressive track record and brings with him strong leadership qualities together with a wealth of experience, which will be key factors in helping us towards our ambitious 2019 target.”
O’Riordan added: “I am delighted to join Relendex during this exciting period of growth, as the firm looks to significantly increase the level of its loans this year.
“I am passionate about this market, and particularly admire how Relendex has stayed true to traditional property values in combination with embracing p2p technology.
“I am confident that my experience and know-how will help the firm achieve the ambitious growth it is targeting.”