Apologies if you've mentioned this source before, but the Brewery History Society had an "Irish special" issue of its quarterly magazine 10 years ago - see details here: <!-- m --><a class="postlink" href="
www.breweryhistory.com/journal/archive/journalcontents.html">
www.breweryhistory.com/journal/a ... tents.html
which contained a county-by-county gazeteer of Irish breweries.
Good sources for 20th century stuff are the pub industry trade magazines kept at the National Library - give yerself a larf and follow the debates as the small Irish pub-based bottlers tried to resist the arrival of filtering and pasteurisation of bottled beers and stouts in the late 1950s and early 1960s, ostensibly because the beer would taste worse (true) but in reality because they couldn't afford the new kit ... Barnard's Noted Breweries also features several of the Dublin and Cork brewers.
You can't beat ploughing through trade directories and back copies of newspapers for gathering info on vanished breweries, and both will often carry old ads which will tell you what beers they brewed. Visiting local libraries is also, of course, essential (and it's amazing what they sometimes have - the Dungarvan museum has a BEAUTIFUL old St Brigid's Well brewery mirror.
A couple of passing points while I think of them: the St Stephen's Green bwy was where UCD later was/now is, I believe - if my recall is correct, the Sweetmans bought it off the Leesons after whom Leeson Street is named (the Irish National Gallery had an exhibition six or eight years ago showing some of the loot the Leesons collected after their brewing-based wealth elevated them to the aristocracy). The Sweetmans had at least a couple of premises going at the same time at one point in Dublin, I think.
Deasy's of Clonakilty is ??still?? a mineral water manufacturer, and has (or had, when I was last in West Cork) a pic of the brewery on its bottle labels