The network has taken a controlling interest in the previously independent agency, which was founded in 1991 by Chris Cawley and David Nea. The shop's client list includes the Allied Irish Bank and the Irish Defence Forces.
Marketing Magazine named Cawley Nea the Irish Advertising Agency of the Year in 2000.
"We are flattered that Cawley Nea has agreed to become part of the TBWA family, Paul Bainsfair, the president of TBWA Northern Europe, said.
"This means the TBWA brand is now represented in all EU countries."
The move comes a week after TBWA announced it is to open an office in Edinburgh . It is pitching for the Royal Bank of Scotland's ad account.