Alrighty. First, considerable apologies for the delay in replying to this, particularly after I encouraged you to post it as a separate question. The week was busier than expected, and the research wider than expected. Expectations, kids. Don't do expectations.

So, short answer first: Irish cheesemaking is not continuous. There's a gap from about the 18th century to the mid-to-late 20th century, and it's not clear why.

The background is that cheese was, in medieval and very early modern Ireland, incredibly important. The Grand Old Man of Irish Food history, A. T. Lucas, once a Director of the National Museum, devoted a considerable chunk of 'Irish Food Before The Potato' to cheese, and says that "cheese of one kind or another formed a standard and substantial part of Irish diet" (and goes on to name seven different types; fáiscre grotha, tanach, maethal, grús, táth, millsén and mulchán). That lasts to the end of the 1600s.

After that, there is a very peculiar drop-off. Regina Sexton (who has done an awesome amount of research into Irish food history) discusses this in Ireland's Traditional Foods, where she describes a decline from the 17th century. Lucas also talks more about that, pointing out that by the 1770s, there was cheese-making being done only by specialised dairies in Offaly, Wexford and Cork, and quotes a 19th-century writer (apparently called Bincheno, but the PDF I have hasn't the most legible footnotes) as saying that 'Cheese is not an article of Irish produce; it is brought to the tables of the affluent as an indulgence'.

And then, as far as I can make out, cheese doesn't quite vanish, but just isn't a big thing in Irish food, and may mostly have been imported from the UK when used, up into the 1970s. Darina Allen, in Forgotten Skills of Cooking, blames this on the Industrial Revolution, with 'economies of scale, price supports and changing tastes' being the major factors. Allen is a cook, not a historian, though.

In 1965, cheddar cheese was sold over the counter in a small shop in Kilmeaden in Waterford (per the company's own history), and that claims to have been from 'a unique, locally sourced traditional recipe'. If that's the case, then it's one of very, very few that survived, and it's for the characteristically British cheddar, not any of the known Irish cheeses. Kilmeaden is now one of Ireland's biggest cheese brands. Cashel Blue, often held up as a very traditional Irish cheese, dates only to 1984, some years after Jane Grubb went on a cheesemaking course (from L&J Grubb's own history). Interestingly, she made and sold quark before she went on to solid cheeses, and yet you still couldn't easily get quark in this country until a couple of years ago. A Teagasc publication on the end of agricultural quotas says that "when the milk quota was introduced [in 1984] Irish cheese production was limited to cheddar and processed cheese". The processed cheese was probably Galtee, which was an offense to the word cheese.

What could be held up as the main 20th century Irish cookery book, All In The Cooking, by Josephine B. Marnell, Nora M. Breathnach, and Ann A. Martin, was a home economics textbook for schools. It references 'cheese' in a number of recipes, so we can assume that some cheese was available - but as far as I can establish, it was not made in Ireland. A recounting by Dr. Alf Delaney for provisioning of an Irish yacht in the 1948 Olympics included '5lb Caerphilly Cheese; 2 Boxes Three County Cheese', and indeed, in the early 1900s, Findlaters' grocery sold 'cream, Cheddar, Gloucester, stilton, gruyère, parmesan, camembert and Dutch' cheeses. And back as far 1830, they sold Dunlop cheese, imported by the barrel.

But why there was that gap in Irish cheese production - when consumption didn't stop! - for two centuries is currently beyond me. More research needed.

Sources:

Trevor Donnellan, Thia Hennessy and Fiona Thorne, The End of the Quota Era:A History of the Irish Dairy Sector and Its Future Prospects

Darina Alllen, Forgotten Skills of Cooking

Josephine B. Marnell, Nora M. Breathnach, and Ann A. Martin, _All In The Cooking

Cathal Cowan & Regina Sexton, Ireland’s Traditional Foods (Dublin 1997)

A. T. Lucas, 'Irish Food Before The Potato', Gwerin vol. III No. 2 (1960) 8-43

Alex Findlater, Findlaters: The Story of a Dublin Merchant Family 1774-2001