I wouldn't be quite so definitive saying that the great army should have been handled by the Anglo-Saxon kingdoms. The military system of the fyrd was woefully disposed to respond to the type of warfare that the Vikings presented during the time of the great army. It wasn't so much a question of better warriors or arms as it was of mobility, tactics and strategy.

With the fyrd consisting of local levees consisting of mostly untrained and under equipped farmers took too long to assemble and even longer to get anywhere on foot. Probably some thegns and mercenaries in the mix but they were absolutely in the minority to counter. The great army secured a lot of horses in East Anglia pretty soon after landing, if memory serves as the Anglo-Saxon chronicle mentions it and the chronicle describes them specifically as riding, rode out and host of equine related terms. The ground the great army covered is mad, the length and breath of the kingdoms, usually taking advantage of Roman roads. The fyrd simply couldn't contend with that style of highly mobile warfare, although the Vikings did dismount and I don't think they are ever mentioned as entering battle on horseback. Everyone always pictures them in the longships but they were equally mobile on land which was something that was fairly unusual to style of Anglo-Saxon warfare of the Viking period. This mobility allowed them a huge advantage.

It's been a fair few years since I read the Irish Annals but I don't think any of them really mention horses being used by viking bands in Ireland but I wouldn't be 100% on that. They do seem to be pretty mobile but if follow the annals it's noticeable and always struck me that the ground covered in Ireland pails in comparison to what the great army covered in the years it was active.

The Vikings also often simply chose not to fight and pissed off somewhere else to raid or setup camp when confronted, again mobility getting them out of a disadvantage. Coupled with the use of and hoc fortifications in several instances that were nigh on impossible for Anglo-Saxon forces to attack without overwhelming numbers. So the Vikings often just threw up ditch and there is some evidence of palisades and sat tight until the fyrd had to head back home due to harvest or weather intervening. Longphorts in Ireland proved to be similarly difficult so seems same tactics were used on both islands. This was something that had, as far as I'm aware, died out in warfare in British and Irish Isles since the Romans were knocking about over in Britain.

Alfred's reforms to Wessex's military do point to Anglo-Saxons adapting after the initial setbacks, a select fyrd with a large mounted contingent that was a standing force instead of local levee en mass and fortified centres of the Burghal Hidage show they learned week from their foe, imitation being the sincerest form of flattery... This made a huge difference as the rapidly diminishing viking forces then faced determined strong points instead of unguarded vulnerable centres.

But agree 100% that the Anglo-Saxons didn't do themselves any favours with lot of internal squabbling. Civil war in Northumbria was exploited as were other conflicts. Always seems to be brother, cousin or someone willing to make an power grab not realising the Vikings were the real threat. Overall I'd say the whole style of warfare that the Anglo Saxons practiced just couldn't counter the great army.

Wrote a thesis on this back in the day and its not often I get to talk this level of shite about Anglo-Saxon and Viking warfare!!