Walsh has been appointed deputy chair, and is among three new additions to the board.
Also joining Walsh are new non-executive directors David Barger and Conor Kehoe.
CarTrawler said the trio would bring decades of experience across aviation and technology.
Barger co-founded US carrier JetBlue, while Kehoe brings experience of working with McKinsey.
Patrick Kennedy, CarTrawler chair, also confirmed the departure from the board of Bobby Healy, who is standing down after 16 years.
Walsh stood down as IAG chief executive earlier this month after nearly a decade in post.
CarTrawler operates the Holiday Autos brand, and has more than 100 international airline partners.
