Listed in London, WYG said it has initially paid almost 8.7m in cash and over 5.3m in stock for the firm, which has offices in Cork, Dublin, Limerick, Kilkenny and Waterford.

WYG said it will pay an additional 1m in cash after 12 months for additional net assets.

Further consideration of 4m in cash or stock will be payable if certain performance criteria are met within the next year's trading.

The Ryan Nolan Partnership generated pre-tax profits of over 2.2m last year on turnover of 7.8m, according to WYG.

The acquisition means a generous payday for Nolan Ryan's seven partners, which include founders Paul Nolan and John Ryan. They have all agreed to stay with the company, which employs 72 staff and has worked on projects for clients such as CIE, Arcon, Deutsche Bank and Bank of Ireland.

White Young Green has aggressively expanded its Irish business base in the past number of years, making acquisitions both in the North and in the Republic.

Much of the buying spree has been spurred by government spending on the National Development Plan, as well as the buoyancy of the domestic construction sector.

Including its latest acquisition, WYG has spent in the region of 40m acquiring businesses in Ireland in the past five years.

With a market capitalisation of £161m ( 232m), WYG said last week that it currently has an order backlog of £300m ( 433m) as a result of recent acquisitions and new contracts, including an order for the 2012 Olympic Games in London.