Hanover has offices in London, Dublin, Brussels, Dubai and Abu Dhabi.
Its Irish arm is headed by Lorna Jennings.
Following the acquisition, Avenir will have around 1,000 employees in 25 locations across North America, Europe and the Middle East.
Its yearly revenue will be approximately £125m (€146m).
Hanover Dublin was established in 2016 under the leadership of Ms Jennings.
Since then it has experienced rapid growth, working with multinational clients in the fields of corporate affairs, government relations and strategic communications.
Commenting on the acquisition, Ms Jennings said: “This marks a significant milestone in the evolution of Hanover Dublin.”
“As the only Irish office in the network, we see substantial growth opportunities for the business here as well enhanced capabilities to deliver the highest level of strategic support for our clients on a truly global scale. I’m delighted to be part of the international scale-up and look forward to the exciting times ahead for the team and the business”.
Founded over 20 years ago in London by current CEO Charles Lewington, Hanover’s clients include Goldman Sachs, Airbus, Microsoft, Apple, Warner Media, and Sky.