Willis has acquired a majority shareholding in Irish broker Coyle Hamilton. Terms of the deal were not disclosed.
It said it would acquire the remaining interest over the next several years.
Coyle Hamilton has annual revenue of approximately €50m, with approximately 500 employees based in offices in London, Dublin, Limerick, Cork and Belfast.
The firm will be renamed Coyle Hamilton Willis.
Willis' existing Republic of Ireland operations will be integrated with Coyle Hamilton once the deal is completed. The Belfast and London operations will be integrated with Willis' operations in those markets.
Coyle Hamilton chief executive Hugh Governey will head up the combined operation in the Republic of Ireland, while Willis Northern Ireland chief executive Jim Halliday will lead the combined Northern Ireland operations.
Willis chief executive and chairman Joe Plumeri said: “This acquisition is consistent with our growth strategy to secure leadership positions in key markets and reflects the Willis model of combining access to global resources with local expertise and close client relationships.”
The deal is subject to shareholder and regulatory approval.