Irwin, who has worked for the WPP-owned Maxus since its launch two years ago and was previously the joint managing director of BJK&E, will continue to report to the chief executive, Lindsay Pattison.
He will focus on all operational aspects of the agency and growing its range of services with an emphasis on data and search.
Maxus is looking for a managing director to replace Irwin.
The agency has also hired Mark Bauer, a founding partner of the now-defunct Polestar Communications, to run its global Barclays account.
Wins including Barclays and SC Johnson have helped Maxus to almost double in size during the past year to 120 staff.
Pattison said: "This will allow us to continue our fantastic momentum in new business and to further our data and digital services."