I'm going to try for a cohesive answer, but I've got a raging migraine, so pls excuse if it's a bit scattered and the spelling is even worse than usual.
First off, these is a very old myth (from the nationalist days at the end of the 1800's) that there was such a thing as a pan-Germanic, cohesive culture. Today, we can be pretty certain (I would even go so far as to say entirely certain) that this just was not the case. What that means is, that how your Germanic world looked depended entirely on when and where you lived. There were vast differences between each area, and the Iron Age spand 1000 years, with the Viking Age only being the last little bit of that time span. Think of it this way: we are as close in time to the vikings as they were to the beginning of the Iron Age, and you muste realize that even though our culture has evolved much faster in the last 200 or so years than they did in 200 years, that timespan is still very considerable. So what Tacitus says in Germania (still counted by many as one of our most prominent sources on "Vikings") has about as much to do with vikings as what the Heimskringla has to do with Swedes today, if you get my drift.
Now, what this has to do with the Irminsul: If there is no cohesive pan-Germanic culture, there is no pan-Germanic politic system. The politics were based on what suited each chief ATM, not on religious cohesiveness. You have to remember, that these people did not even have a word for their religion. It was the Christians who, much later, named it. Unlike Christians, they never tried to convert by way of the sword, and when other faiths attacked in the name of their faiths, they fought back in the name of their chiefs. Now, that is to do with the religion, because the lord was also the godi, who was in charge of communal religious events. BUT it still is not the same as jihad. The Norse fought for silver and silver lead to politic standing. Politic standing was often proof of the gods' favour. What that means, by extension, is that just becasue you pissed of one heathen, you were not automatically the enemy of all heathens. If the heathens of Norway even knew about the Irminsul being distroyed, it may not necessarily have meant diddley to them, because their own rituals differed so much from those of the German heathens. I am a heathen, too, and frankly, I don't give a toss about the Irminsul any more than any other tree, because I believe that we already are in the world tree. The Germans had a ritual involving a specific tree in a specific place, but to me, the trees in Uppsala were holier in that case. They were part of my people's ritual. We were and are different people with different politics and different ritual practise. Just because the Christians upset one heathen, that doesn't mean they upset his neighbour. Actually, his neighbour, who didn't recognize the Irminsul as anything special, might have thought it was a great thing, since it may have located a chink in the other tribe's armour! That's a good time to raid, before the other tribe figured out just exactly how the destruction of the Irminsul affected them mentally, and if it had damaged their relationship with their gods.
What the Irminsul has to do with the start of the Viking Age is, that it was one of the first symptoms of things to come. It was the first time that a Christian king had made such a concerted effort to squeeze upwards towards heathen territory, outlawing trade with pagans, and thus making the previously flourishing trade (which is what carried the political system in Scandinavia) impossible. It put an end to the previous system of raiding, and made it so that the Scandinavian people, instead of going south for trade and political alliances like they had for several millenia previous, got squished out towards the sides. They couldn't go north and they could no longer go south, so they went east and west. The destruction of the Irminsul was a symptom of the beginning of a disease called Christianity. Similar symptoms cropped up all over after that, but the events that set that off had nothing to do with the Irminsul, but rather, with the goings on within Christian areas and the politics of the Roman-Catholic church.