[P.S. added at the top where it will do the most good. Uh-oh. Looks like this post is going to go on a while, and not be concise at all. ;')]
The Vikings were the direct result of the Medieval Warming, and were going great guns for quite a while. Generally their decline is dated to Stamford Bridge, where Harald Haardrada (sp varies) got croaked by Harold II of England just weeks before William the Bastard invaded.
During the House of Normandy (I think that was the name, I used to know this stuff; four kings I believe, W, W2, Henry I, Stephen; the Normans of course being of Scandinavian descent), the Kings of England started carving chunks out of Ireland's eastern coast, I believe it was called "The Pale".
Sometime prior to the first English presence, the former Caig Caigi (five fifths I think it was; the five kingdoms, Leinster, Munster, Connacht, Ulster, and Meath) had become four kingdoms, when Meath was dismembered. I dunno for sure, but it's possible that internecine warfare and who knows what else weakened Ireland.
The same thing happened to Celtic areas in Britain; constant struggle between the Welsh and their lifelong blood enemies, the Welsh, led to continuous English rule from the time of Henry VII to the present day. Scotland was purchased piecemeal (bribery). And sometime since the '45 (1745, Bonnie Prince Charlie) someone wrote a tune "Parcel of Rogues" (recorded by, hmm, Steeleye Span I think, and an obscure group called "By Blood and Marriage") referring to the English. ;')
I've got bloodlines from all over the British Isles. (':
Like England, medieval Ireland was probably dotted with villages. Sites of Roman towns still were occupied in England, but most of those didn't resemble the Roman period original, the originals having been pulled apart for other things. I don't know that a single Roman-era bridge survived, and a bridge is a weird thing to pull down IMO.
The town name where my surname ancestors last lived before taking the boat over here 370 years ago can be found in the Domesday Book, but like a lot of those place names, it was a hamlet. :')
"the Normans of course being of Scandinavian descent"
Only partly.
They were French, a blend of the Gallo-Roman Armoricans, who had been there for centuries, and the Norsemen who settled in Eastern Armorica starting about two centuries before the conquest of England.
It would be more accurate to describe the Normans as "Celto-Norse Frenchmen". But for some reason (not very hard to discern), Anglo-Saxons prefer to insist rather heavily on the Scandinavian origins of the Normans. (To wit: it is galling to say that England was conquered by the French. It makes it better to say that England was conquered by the Vikings. One finds a similar prejudice in the 19th Century linguistic efforts made in England. So much of the English Latin is said to come from Latin. That is true, but in the same sense that America has been settled primarily by the Goths. The words didn't come into English from the Romans. They came into English from Old French, the language of the Conquest. Old French, of course, largely came from Latin. Now, one can say, if one wishes, that "half of English comes from Latin". But that's rather indirect and deceptive. A truer statement would be that half of Middle (and Modern) English comes directly from Old French, and that French derives mostly from Latin. Politics and prejudices shape even the way we look at history, if we're not careful.)