Gahan and Long, based in Belfast, will contribute to the project environmental impact report as part of the application process for the development. The application is expected to be submitted in 2016.
The archaeological firm will examine what impact connecting the offshore wind site to the Northern Irish grid network will have on historic and cultural heritage features.
Their findings could affect the design or location of the onshore landfall, cables and substation.
First Wind, a consortium of B9 Energy, Dong Energy and RES, had to cut the proposed wind project from 600MW to 300MW-400MW in June. The reduction was aimed at reducing the impact on shipping routes and fishing grounds.
The planned site is located eight kilometres off the County Down coast in the Irish Sea and is due to be operational by 2020 if it obtains all necessary consents.