Its worth considering that the Irish consistently pasted the Vikings after the initial shock of their arrival.
There was no shortage of full time warriors in Irish society at this time and the nobility who they served were expected to excel in warfare.
For a big battle like Clontarf the free farmers would turn out too in accordance with their obligation to take part in wars of offence and defence. They would all of had some degree of regular military experience. Irish Tuatha hosted on an annual basis.
The Norse resident in Ireland had been under Irish military domination for some time including sending contingents for their overlords wars.
I think they would roughly approximate in quality to their Irish opponents.
Irish skirmishers were warriors tasked with a job rather than troops who only skirmished. How they fought was dictated by the circumstances of the encounter. Sometimes they skirmished sometimes they charged.
The external allies called in by both sides add a bit of variety.
It might be interesting to model the various commands on both sides.
As has been said Clontarf was a very bloody affair.
