In October, Grant Thornton Ireland and Grant Thornton US confirmed they were merging their advisory and tax businesses, in a deal backed by New Mountain Capital. The New York private equity firm had previously taken a majority stake in the US affiliate. The tie-in with the Irish firm quickly followed, and the deal is expected to be rubberstamped in the first quarter of next year.   “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,” Steve Tennant,…