As far as my knowledge of the viking sttlers goes they behaved in a very similar to the Moors in Spain.

They came, they conquered, the settled and then they let people prety much carry on as usual. Dublin was just a trading post for them really that they commanded.

No religion was imposed. No lifestyle changes were enforced except for 'you must sell/trade with us.

From what I know they didn't really travel much inland nor did they mix in ... erm ... a genetic way.

I believe this to be he case in many other parts such as the Scottish islands. I remember the same being said of York too.

correct me if i'm wrong(and ignore the following blab if so) but has most of the DNA work on vikings not been done using the Y chromosome and thus following the male line. This technique assumes that only the viking men passed on their genes to locals and of course doesn't include any females born of a viking father.
If we look at a more settled society assuming viking woman came along too (as i think some did). Then if Mr and Mrs Viking had a daughter who then married a local the original viking mothers mitochondrial DNA would be passed on ( the fathers mitochondria, although in every cell,are only used for motility of the sperm and are never carried on) and thus vikingness would be seen in the female line assuming there are specific differences between viking mitochondrial DNA and native DNA.
I think the two techniques together would give a clearer picture of settlement overall. It is quite likely that when settled in the mainland men went off to fight and were killed leaving women behind that would have carried on the genes.
Both men and women could be tested in this manner as the mother passes the mitochondria to both sexes although as stated the men don't pass it on.
Anyway it is a more complex technique than Y-chromosome testing i think but may shed some more light on the situation.
maybe it has been done!
love
carolinex