PHMP is one of Ireland's leading consultants and specialises in large public infrastructure projects in the water and highways sectors.
Its 184 staff will become part of WYG Ireland with all company directors joining the WYG Ireland senior management team.
WYG chief executive Lawrie Haynes said: "The P H McCarthy team will give WYG Ireland a significant boost in accessing the strategically important public infrastructure development market both in Ireland and internationally. It also balances and complements the existing WYG Ireland engineering business."
P H McCarthy has been involved in major projects across Ireland including Sillogue Water Tower; the M3 Clonee-Kells Motorway and the N25 Waterford Bypass.
White Young Green Ireland now employs over 670 staff.