They were a Dublin-based malt roaster. And, most importantly, pretty much house roaster to Guinness.
As this testimonial attests.
James's Gate Brewery, Dublin, 25th February, 1873
"Dear Mrs. Plunkett,
We have the pleasure of stating that we are purchasing considerable quantities of your Patent Brown Malt, and find it of very good quality. We would alio say that our connection with your Firm and that of your husband, the late Mr. Randal Plunkett, and his father, extends over fifty years, during which we have had large and satisfactory transactions.
ARTHUR GUINNESS, SONS & CO.
Mrs. Eliza Plunkett, Belle Vue.
The above text comes rom a Plunkett Brothers advertisement in "The Theory and Practice of Modern Brewing" by Frank Faulkner, 1888. By which time Guinness hhad probably swapped out the Patent Brown Malt for roast barley. I know for certain that they were using roast barley in 1894, because I've seen it in a bbrewing record.
Plunkett Brothers produced an impressive range of roast grains:
Chocolate Finest Patent Malt Roasted for Flavor.
Finest Patent Malt Roasted for flavor and color.
No. 1 Patent Roasted Black Malt for color.
No. 2 Patent Roasted Black Malt for color.
Patent Roasted Barley.
Patent Roasted Maize (Patent granted June, '80)
Special “Candied” Malt (Registered March 31, '85)
Golden Finish Malt.
No. 1 Amber Brown Malt.
No. 2 Amber Brown Matt.
High-dried Pale Malt.
I wonder what roasted maize was like? And what the hell it was used for.