He will replace Roger Pratt, who is stepping down after 12 years in the role as managing director of the body that audits newspaper and magazine readerships.
The search for the chief executive post was undertaken by NRS chairman Simon Marquis, headhunter Bird & Co and the NRS board, which includes Paul Hayes, managing director at News International Commercial and Caroline McDevitt, outgoing IPC Advertising managing director.
Ironside will take up his role in January, although Pratt will stay on for six months on a consultancy basis, with the possibility of a longer stay.
Ironside, who had been with Associated Newspapers for 14 years, left The Mail on Sunday in October 2003, after five years as managing director.
His previous roles included ad director and client advertising manager for the Daily Mail.
Since leaving The Mail on Sunday, he has worked in a consultancy role for Polish media outfit Agora Group.
He also set up So London, the upmarket weekly magazine that launched last year, but closed after two issues.
News International's Hayes said Ironside was a "well-established" media figure and claimed he would be able to "get consensus" on key issues facing the newspaper sector.
The appointment of Ironside comes at a crucial time for the NRS, which has been criticised in some quarters for the methodology it uses to collect data for its readership estimates.
Ian Clark, managing director of thelondonpaper, believes the body needs to increase its sample size and include online surveys to reach young upmarket Londoners.