IRI said McNamara decided to leave following a review of the UK business's strategic direction and an assessment of key opportunities and management imperatives.
Prior to becoming MD of the UK business in 2006, McNamara was the finance controller for Europe.
McNamara will be succeeded by Ruelens, who had full general management and cross functional responsibility as global head of the 16,000 strong beverage and confectionery sales offering at Cadbury.
Prior to joining Cadbury, Ruelens held senior sales positions with General Mills, Pillsbury and XL Foods, having started his career at Procter & Gamble, where he served in a number of sales and marketing roles over 17 years.
Nigel Howlett, international president of IRI, said: "I am delighted to have hired an executive of such outstanding calibre. The UK market presents us with a wide range of opportunities which we need to swiftly develop.
"Blair brings us the experience and expertise to lead and build the UK business in a truly client centric way."