The Romans never invaded Ireland. They claimed it wasn’t worth it. It’s true that Roman soldiers viewed living in Britannia, even Londinium, as a punishment. (“They eat gross stuff like beef, cheese, and beer! I want my wine and olives!” “These barbarians wear pants! Why can’t they be civilized and wear togas?” “I’m too far away! I miss my home!) But they certainly loved some of the resources from Ireland, especially the wool cloaks and the wolfhounds. Likely, so much of Ireland, especially central Ireland, was filled with people expert in making war, especially guerilla tactics, that the Roman military leaders determined that what they could extract from the country might not pay for the blood lost in obtaining it. As a result, Ireland didn’t adopt the Roman market-town system or its administrative system.
(Life lesson from when the Romans left: if you suck at defending your country from invaders, don’t ask a guy that’s stronger than both you and your invaders to help you out. Those Angles, Saxons, and Jutes took the invitation to row over to Britain, then they checked things out, and then they wrote back to their buddies: “These guys are wimps! Come join us and we’ll take over everything!” Which they promptly did.)
Back in Ireland, the Catholic church (practiced with a distinctly Irish flavor) provided an administrative system. It also created locations with a concentration of wealth and knowledge in the form of monasteries (called that whether the inhabitants were all-male or all-female). Because there were no market towns to ransack, these religious centers became the targets when the Viking invasions began. A poem from this era roughly translates as: “The bitter wind howls tonight/Tossing the waves until they’re white/Tonight I can sleep peacefully/No Vikings sail on the Irish Sea.”
The tactics of the Viking sea-kings were to slip in, grab all the gold and people (especially children and women) they could to sell as slaves, kill anyone who stood in their way, and disappear on the ocean in their technologically superior long-ships. There were two main routes to the slave markets of the Mediterranean– people captured on the west side of northern Europe were sold via Spain, and people captured on the eastern side of northern Europe (i.e., Slavs) were taken down river to Constantinople for sale to the Muslims. The Vikings were so prolific at this for more than two centuries, that the name of the Slavic people became synonymous with “slave”– literally. After their largely successful attack on the famed fortifications of Constantinople, the Vikings had the respect of the people there, and they were eagerly hired as mercenaries.
Vikings demanded so much silver for “protection” money that it bankrupted several kingdoms. At one point, Vikings went on a lark to Paris, took over everything for a few months while jeering at the powerless Parisian leaders, partied like crazy, and then just left when they’d used it all up and felt like going home. One king got the bright idea of giving land to the “North Men” (i.e., Normans) (i.e., Normandy) (i.e., the grandpas and grandmas of William the Conqueror) in exchange for their agreement to fight off other Viking invaders. When it came time to pledge fealty to the king, they told the Viking leader he needed to kiss the king’s foot. The Viking leader raised an eyebrow at the diminutive king, then ordered one of his war-captains to do it on his behalf. The war-captain reached down, grabbed the king’s foot, and possibly planted a kiss on it as he tossed the king on his backside. And that pretty-much set the tone for the relationship from then on.
Eventually, these Vikings turned into landed nobles. Soon after William conquered the Angles, he began building fortifications which were the predecessors to stone castles. The style caught on, and soon everyone wanted one.
Side note: You won’t find a castle on top of Benbulbin. The castle described in the Marcel Cycle of the “Stasis” series is a result of an interference in the flow of history. But it is architecturally appropriate for its time.