But the Normans did not penetrate very far inland, as this map from the book demonstrates:
Essentially, there is an arc of settlement from Dundrum Bay through Downpatrick and Lecale up around the Ards Peninsula through Bangor, petering out around Belfast (but not up the western shore of Strangford Lough); another arc from Antrim Town through Newtownabbey and Carrickfergus up to Larne; and a more diffuse concentration in the northwest Antrim/Coleraine area. And that's it, bar a few outposts elsewhere (Greencastle Co Down, Greencastle Co Donegal, Dromore, dubious religious endowments further in). De Courcy's wife was a Viking princess, and it's impossible not to look at that map and see that the sea connections matter much more than the land links. From the mid-fourteenth century, the only part of Ulster that remained securely under Dublin/English control was Carrickfergus Castle, which is much more easily supplied by sea (much later on, that was where William III landed in 1690).
The story of the Earldom is not just a landgrab by adventurers (of the kind the Normans and their kin were engaged in from Newfoundland to Palestine) then eroded by the natives coming back. For a start, De Courcy was able to exploit a power vacuum when he arrived in 1177, after the local chieftains in Downpatrick had killed each other off. It's an episode which looks rather odd viewed in the context of the rest of Henry II's Irish policy, in that normally the practice was to marry into the ruling clans and demand submission fromt he rulers rather than simply kill them off. I wish McNeill had looked at it in the context of the career of De Courcy's father-in-law, which I think helps make more sense of it. In any case, it's clear that local actors were vigorous behind the scenes in ensuring that De Courcy, a rather young but obviously charismatic man, took and held power along the eastern Ulster littoral until King John got tired of him in 1204.
The Earldom might well have prospered in the long term - it seems to have been economically self-sustaining, and a committed Earl could usually ensured that the neighbouring Irish chieftains would occupy themselves fighting each other - had it not been for catastrophic dynastic failure in the mid-fourteenth century. McNeill discounts previous historians' suggestions that the Earldom was killed off by the devastating invasion of the Bruce brothers in 1315, as documentary records show it was still a going concern for several years after (and also, I would add, that the Scots were no worse at devastating than the various Irish and Norman devastators of the previous 120 years).
Instead he points to the deaths in quick succession of the heir to the earldom, John de Burgh, in 1312; the Red Earl himself, Richard de Burgh, in 1326, and then his grandson via John, the Brown Earl, William de Burgh, murdered in a family feud in Belfast (almost the first thing that is ever recorded to have happened in Belfast) in 1333, leaving the earldom to an infant daughter in the care of his mother, John de Burgh's widow. She was a redoubtable woman who was widowed three times and founded Clare College, Cambridge, but maintaining her granddaughter's inherited property from her own first marriage was not among her priorities, and while the title of Earl of Ulster eventually merged into the royal family (and is now held by the 20th in line to the throne), the lands, apart from Carrickfergus Castle as mentioned above, were left to fend for themselves and mostly ended up back in Irish rather than Norman hands.
Lots more here about architecture and economics (and far more about pottery than one would have thought possible, given that a) there is very little of it and b) it is very boring), but it is inevitably the politics that grabbed my attention.