Dunn will also take a minority stake in The PC Agency, which was created in the summer by Paul Charles after an "amicable demerger" from his business partner and an acquisition of another small agency, Angel Publicity.
Charles, who is currently the outright owner of the business and formerly comms director at Eurostar and Virgin Atlantic, did not disclose the size of Dunn's stake.
Charles described Dunn as an "incredibly switched-on businessman, and very alert to new ways of working in the travel industry", saying that he "built his business over 30 years into a real market leader".
"His business and travel knowledge will be invaluable as we continue to grow in the months and years ahead," Charles said, adding: "Our plan is to continue to grow in terms of quality - we're not trying to be a really big consultancy, we're trying to offer the highest-quality service."
The agency now has 23 staff, having made two new hires since it was founded, and several internal promotions, including Sarah Potter to associate director.
The PC Agency's clients include hotel groups Lime Wood Group, which includes The Pig, and Como, as well as Tourism Ireland, Tourism New Zealand, Visit Finland and new tour operator Red Savannah.