Been reading up a lot on Irish brewing stuff lately.
The commercial hop industry was not established in Ireland until 1961. To the best of my knowledge they only ever had one customer.
Hops were imported from all around the world, but preference was given to English varieties, sometimes as a result of government controls on "foreign" imports, but a lot of Irish breweries didn't use hops at all (and are now gone bust for brewing piss), while a few did grow their own.
Guinness traditionally used 50% American 50% English hops in their stout until their research dept discovered that the "soft resins" were where the bitterness came from for their Stout. From there on they bought based on soft resins.
Guinness dealt with an American hop broker in the early part of the last century by the name of Haas. Haas went on to be part of Barth-Haas, now owner of Simplyhops from whom we have bought hops. Barth-Haas (I think) is the biggest hop company in the world.