WYG believes the firm - one of Ireland's largest indigenous cost management businesses - will increase its management services business in the Republic.
Nolan Ryan works for private and public sector clients, offering quantity surveying, project management, value engineering and value management.
It employs 72 staff and has five offices in Kilkenny,
Waterford, Dublin, Cork and Limerick.
The company made £1.5 million in pre-tax profits last year on a turnover of £5.4 million.
Chief executive John Purvis said: 'This acquisition is a significant step forward in the ongoing development of WYG in Ireland. Nolan Ryan is one of Ireland's largest indigenous chartered quantity surveying businesses.
'It is a successful cost management consultancy which brings with it a strong order book from an established portfolio of clients across both the public and private sectors in the south of Ireland.'In a trading update the company added that its order book had increased 30 per cent to £300 million.
It said: 'Market conditions in the UK, Ireland and internationally remain favourable, opportunities continue to gather pace and the acquisition pipeline is strong.'