Drury, which is already owned by Omnicom and has worked with Porter Novelli previously, is rebranding as Drury Porter Novelli.
With 20 staff, the agency offers corporate, consumer and lifestyle, financial and healthcare comms and public affairs for clients including HP and Nivea.
Drury managing director Anne-Marie Curran will continue in the same role and Portelli Novelli president Sally Ward has been appointed as a non-executive director.
According to Ward this is likely to be the first of several expansions into new regions for Porter Novelli.
"It is critical that we have a network presence in key markets where our clients need us," she said.
"We are seeing more and more opportunities coming from Ireland both in terms of new business and expansion from existing clients. With Drury we have a partner with the experience, expertise and enthusiasm for building business that perfectly matches our own."