The deal includes the Irish advisory and tax business. The agreement does not include Grant Thornton’s Irish audit business which will continue as an independent firm.
The current combined Grant Thornton Ireland has annual revenues of more than €300m.
The US merger will enable Chicago-based Grant Thornton US – which sold a 60pc stake in the business this year to a group led by private equity giant New Mountain Capital – to expand its international business.
During the summer, it was reported by the Financial Times that the US-based Grant Thornton office has held exploratory talks about acquiring the Irish and UK businesses.
At the time, Grant Thornton Ireland emphatically rejected any suggestion that it would be sold.
“We have engaged advisers to assist us in assessing our options,” it said at the time. “Grant Thornton Ireland is not for sale nor engaged in any takeover process.”
On Thursday, the firm said that an agreement had been reached to combine Grant Thornton Advisors with Grant Thornton Ireland’s advisory and tax businesses, with backing from an investor group led by New Mountain Capital.
“Together, the two providers go to market as the Grant Thornton brand in the US — most recently achieving record annual revenue of $2.4bn and operating more than 40 US offices that employ almost 10,000 professionals,” according to the firms.
“This transformational partnership will enhance our appeal to a much broader international client base, as the first truly integrated US and Irish professional services firm, combining the expertise and reach of both firms,” said Steve Tennant, CEO of Grant Thornton Ireland.
He added: “This is a key moment in shaping the future of professional services in a rapidly evolving global landscape, and we are looking forward to partnering together to accelerate growth and deliver impactful benefits for our people and clients across diverse industries and geographies.”
There are about 75 partners at Grant Thornton Ireland, spread across nine offices. It employs about 3,000 people and its revenue performance has strongly outpaced that of the UK arm.
In May this year, Grant Thornton Ireland, backed by its insurers, agreed to pay more than $19m to settle claims made against it in relation to audits of a US business.
Grant Thornton Ireland had been accused of taking part in a cover-up by colluding during audits to help a US investment advisory firm persist with fraudulent activity. It rejected the claims.