Powerscourt has previously worked with Ballymore, providing strategic comms advice at a group level.
The agency will now provide all comms advice and tactical support in the UK, working across the national and trade media.
The account will be led by Powerscourt director and former business editor of The Sunday Times, Rory Godson.
Powerscourt reports to Paul Keogh, group director, strategy and comms. Fleishman Hillard will continue to work with Ballymore in Ireland and Germany.
The firm, owned by Sean Mulryan, is behind some of the most notable developments in Canary Wharf and specialises in large-scale, mixed-use developments in urban areas.
It has principal offices in Dublin and London and a presence in four other EU nations. The company was recently in the news for selling the site for the new US Embassy.