The origins of Grant Thornton’s Ireland merger with Grant Thornton US, announced today, can be traced back to April and a conversation between Steve Tennant and Seth Siegel. Grant Thornton’s US business was in the process of selling a piece of itself to private equity (PE) investors, with New Mountain Capital acquiring a majority position in the firm’s non-audit business. Siegel, the chief executive of the US business and a key architect of the New Mountain deal, asked Tennant, who headed the Irish business, if he were open to a deal.  The two firms were part of the wider Grant…