At that time there were twenty-eight flamens in Britain and three archflamens, to whose jurisdiction the other spiritual leaders and judges of public morals were subject. At the Pope's bidding, the missionaries converted these men from their idolatry. Where there were flamens they placed bishops and where there were archflamens they appointed archbishops. The seats of the archflamens had been in three noble cities, London, York and the City of the Legions, the site of which last, by the River Usk in Glamorgan, is still shown by its ancient walls and buildings.... Parishes were appointed off, Deira being placed under the Metropolitan of York, along with Albany, for the great River Humber divides these two from Loegria. [IV.19]

The area called DEIRA appears first in the Historia as part of the establishment of church government in Britain. Immediately thereafter, it appears as the site of struggles between the Britons, under Sulgenius, and the Roman delegate Severus. These battles lead to the building of HADRIAN'S WALL:

In the end Severus drove the Britons beyond Deira into Albany, where, with Sulgenius as their leader, they resisted with might and main, often inflicting immense slaughter on both the Romans and their own countrymen, for Severus took with him as auxiliaries all the island peoples he could find, and as a result frequently left the field as victor. The Emperor was annoyed at this revolt by Sulgenius. He ordered a rampart to be constructed between Deira and Albany, to prevent Sulgenius from pressing his attack closer home. A tax was levied and they built their wall from sea to sea. For many years afterwards it held back the attacks of the enemy. [V.2]

Geoffrey returns to the wall again in the dying days of Roman presence in Britain, after the Romans have defeated a marauding horde of Picts, Huns, Scots, Norwegians, and Danes:

As soon as the legion had freed the prostrate people from this fearful maltreatment, the population was ordered to construct a wall from one sea to the other, to divide Albany from Deira. This wall was erected by a great crowd of workmen, as a deterrent which should hold off their enemies and as a real protection for the inhabitants of the country. Albany had been completely devastated by the barbarians who had landed there, for when hostile people came they looked upon that country as a convenient lurking-place. The local inhabitants therefore pressed on with the work and finished their wall, using private funds and public ones which they had collected. [VI.1]

Deira and the wall appears again in Hengist's pact with Vortigern:

"Let us invite my son Octa to come here, and his younger brother Ebissa, both of them distinguished warriors. Given them the lands which are in the northern parts of Britain, near to the Wall between Deira and Scotland. There they will withstand the onslaught of the barbarians." [VI.13]

Tatlock sees no exact knowledge in any of this, pointing out that Geoffrey's Deira, essentially the north of England, extends all the way to Scotland, when in fact it never did extend this far north (p. 9). As for the wall, Geoffrey does not know that there were two Roman walls, the first built by Hadrian from the Solway to the Tyne, and the second under Antoninus Pius from the Clyde to the Forth (p. 10). Geoffrey's main source is Gilda's De excidio et conquestu britanniae:

15. The Britons, impatient at the assaults of the Scots and Picts, their hostilities and dreadful oppressions, send ambassadors to Rome with letters, entreating in piteous terms the assistance of an armed band to protect them, and offering loyal and ready submission to the authority of Rome, if they only would expel their invading foes. A legion is immediately sent, forgetting their past rebellion, and provided sufficiently with arms. When they had crossed over the sea and landed, they came at once to close conflict with their cruel enemies, and slew great numbers of them. All of them were driven beyond the borders, and the humiliated natives rescued from the bloody slavery which awaited them. By the advice of their protectors, they now built a wall across the island from one sea to the other, which being manned with a proper force, might be a terror to the foes whom it was intended to repel, and a protection to their friends whom it covered. But this wall, being made of turf instead of stone, was of no use to that foolish people, who had no head to guide them.

18. The Romans, therefore, left the country, giving notice that they could no longer be harassed by such laborious expeditions, nor suffer the Roman standards, with so large and brave an army, to be worn out by sea and land by fighting against these unwarlike, plundering vagabonds; but that the islanders, inuring themselves to warlike weapons, and bravely fighting, should valiantly protect their country, their property, wives and children, and, what is dearer than these, their liberty and lives; that they should not suffer their hands to be tied behind their backs by a nation which, unless they were enervated by idleness and sloth, was not more powerful than themselves, but that they should arm those hands with buckler, sword, and spear, ready for the field of battle; and, because they thought this also of advantage to the people they were about to leave, they, with the help of the miserable natives, built a wall different from the former, by public and private contributions, and of the same structure as walls generally, extending in a straight line from sea to sea, between some cities, which, from fear of their enemies, had there by chance been built. They then give energetic counsel to the timorous native, and leave them patterns by which to manufacture arms Moreover, on the south coast where their vessels lay, as there was some apprehension lest the barbarians might land, they erected towers at stated intervals, commanding a prospect of the sea; and then left the island never to return.

There are several useful sites associated with Hadrian's Wall: Hadrian's Wall Country, the BBC's Hadrian's Wall Gallery, and the BBC's Housesteads Roman Fort virtual tour.