Zombie thread, coming through!
I was at a talk last week by the Guinness archivist about their collections and services. I asked her afterwards about the defunct breweries that Guinness took over. She said in most cases Guinness didn't buy the company, just the site, so would never have received the company records. In the case of Macardle Moore, Diageo donated the papers to Louth County archives so they're there if anyone wants to look at them. But things like the pre-Guinness records of Great Northern in Dundalk or O'Connell's in Dublin were never part of their collection.
She denied that Guinness ever kept an equivalent of the Whitbread Gravity Book: the vital statistics of competitors' beers. I find that a wee bit hard to believe, but there you go. Mind you, 45% of the collection is still uncatalogued so it might be in there somewhere among the 3000 ledgers.
The big thing they do is genealogy work as they have the full personnel records back to about 1880. Largely because of this they operate a 30 year rule on all documents, so you might be able to get the recipe for Guinness Lite but will have to wait until 2027 for Dark Angel Lager <!-- s:P --><img src="{SMILIES_PATH}/icon_razz.gif" alt="

" title="Razz" /><!-- s:P --> Some documents older than 30 years are still classified as commercially sensitive and aren't made available.
Anyway, if you're interested in this sort of thing they are open to the public by appointment. The website is here[/url:1quwuu5w].