Guinness seems to have started using some American hops, sourced from English merchants, as early as the 1870s. In 1906, soon after Horst’s first delivery of hops from Oregon and Washington, the total American hops was 39,778 zentners, while English hops provided 33,425 zentners. I guess these are standard zentners of 50kg.
Thanks to Peter Kopp for sharing some of his references, especially Guinness and Hops by J.F. Brown (1980).