A problem I see w using M222/L21 ratio is that it was likely a much different ratio back in the 800’s 900’s when Norse were taking prisoners in raids. If M222 is as believed, to be a gene spread by upper class nobles, 2 problems arise. 1. It was most likely less frequent in the era of Viking raids 2. in Gaelic societies the upper class w the exception of clergy, were ALL warriors/protectors of the people, so capturing or taking them prisoner was far less likely than say a pig farmer or cowboy unless it was children or clergy.
M222 were supposed to be from the upper class of N Ui Niall (also possibly Connacht kinsmen) so they are located in Ireland where those clans/tribes were known to be i.e. the Northwest, west. This spread is said to have happened from their being preferable mates because of status (Gaelic marriage practices also would have played a large role). Since today they are up to (IIRC) 20% or so of N Ui Niall names it is likely they were far less in previous generations. In Scotland they would be located initially in the western coast/isles in the form of allies, mercenaries, of Dal Riata, ( The kinel Connaill kings were generally allies of Dal Riata, so intermarriages, mercenary work, military assistance etc are quite probable avenues for M222 spread). There would also be clergy n laymen connected to Columba/Columcille abbeys. These would later move inland to Athol because of raids.. Eventually this branch would give the Scots their kings from 1030’s to late 1200’s. At that time branches of the family were said to have spread far an wide as upper class noblemen from the very N (McKay rulers) to Cumbria (Washington’s for instance). So you would not expect to see many as prisoners except, initially clergy such as from attacks on Iona or children and M222 would have been a very minor portion of Scots until the 1000’s so you would not expect them to pop up in large numbers among prisoners taken to Norway.
Also keep in mind L21 can come from S Britain or elsewhere as well where there would likely be very little M222 (try telling the percentage genes of Anglo Saxons taken as slaves apart from Dane or Norse genes)