What were the different ways it was mined and how did locally mined iron stack up to other areas in that time period (quality and quantity wise?). Do we have any idea how much was imported/exported from the area? How did the availability and quality of iron affect life in terms of agriculture/war etc compared to other European areas?
Scandinavia was producing iron on an almost industrial scale during the early middle ages. Poland was too, Germany a little less so (I believe).
They used a technique called direct reduction or bloomery smelting to turn ore into iron. Like /u/mightyflynn said, much of their ore was from bogs, which was then mixed with charcoal and packed into a clay cylindrical furnace that was about a meter tall. The charcoal was lit and, as it burned down, more was piled in on the furnace's open top. There was a small hole near the base of the furnace to which a bellows was attached to force in air. The furnace burned for hours, getting hot enough to melt the sand in the ore which turned to glass and slid to the bottom of the furnace, forming a molten bowl-shaped lump of slag. The iron particles from the ore slowly slid down through the burning charcoal and collected inside this bowl of molten glass. The glass acted as a flux, reducing the point at which the iron particles could be sintered together, so they stuck together in a big lump of glassy metal at the furnace's bottom. Once this continued for the better part of a day, the lump of metal (the 'bloom') was pulled out of the furnace, hammered to squeeze out as much of the molten glass as possible, and often split open with an axe to make the inside visible and confirm that the iron particles had stuck together properly.
This process rarely reached temperatures hot enough to actually metal the iron - instead, the particule were simply welded together. As a result, they tended to be full of a lot of impurities, and the bloom's composition was inconsistent. Smiths would usually have to hammer and fold the metal repeatedly to regularize its composition before using it. Some parts of the metal would have a lot of phosphorus in them (an element especially common in bog iron), which would make them easier to harden, more brittle, and incapable of carburization (could not be turned into steel). Some parts that were low in phosphorous could be carburized by the charcoal and might be good quality steel. Most of the bloom would, in the majority of cases, have been a mix of various contaminates that ensured it would be pretty sturdy but very difficult to harden into a good tool edge. This iron wasn't the same quality you see in Asia during the same period, though by the end of the early middle ages it was becoming more common to have access to steel of consistent and good quality (judging by metallographic analyses of surviving artifacts in England, at least).
It's hard to tell precisely where iron was made. Impurities in the metal result from the specific ore used to make it, but the chemical processes are still not completely understood. So it's often not possible to prove whether iron used for a speciic tool was imported or not. However, some of the large smelting sites in Scandinavia during the early middle ages were clearly producing a surplus, and it's very likely that this iron was being exported.
There's also good evidence for some level of local production in most regions. A recent study of iron distribution in graves in England found that areas with more access to iron ore tended to have more iron objects discarded in people's graves. That suggests, the study argued, that people who had good access to ore also had enough iron to throw some away - ie, they were probably doing a lot of smelting. The smelting furnaces can be hard to find, though, as often the only thing left is a metalic lump of glass buried in the ground, and most metal detectorists don't bother to try to dig up faint traces of iron (since there's so much iron in the ground, most of which is of no archaeological value).
We're continuing to make new discoveries of medieval iron - they recently found an iron heavy plough at Lyminge (a 7th century high-status site in England), which must have represented a small fortune in metal. This is the earliest plough of this kind found in England, and suggests that agriculture may have been making use of bigger, heavier ploughs in the earlier middle ages than we'd previously thought.
For everyday use, iron tools' quality could be hit and miss. Because the materials were inconsistent, you had some crappy tools whose iron wasn't as good as the smiths thought when they were making it (you can see the smiths trying to harden tools sometimes, but there's just not enough carbon there to make it work). Other times, people clearly settled for poor-quality soft iron in their knives, presumably because they couldn't afford better. This was pretty much the norm through the Roman period as well - the Roman empire producted massive amounts of iron on a thoroughly industrial scale, but the quality wasn't especially good. Iron seems to have become a little more scarce in the early middle ages (outside of regions like Scandinavia, which was producing a surplus), and its quality didn't improve. But by the 7th and 8th centuries, this was starting to change, and tools show marked improvements in quality - there's something of a minor revolution in iron making technology.
This translates to weapons as well. Swords and spears from the Roman period through the early middle ages were generally somewhat inconsistent and unreliable, but they improved during the end of the early middle ages. Many weapons were clearly made from local iron (recycled pieces, or low-quality iron of the kind that would have been made from local ore), but by the 7th century you start to see some really high purity metals showing up in England that are probably being imported from Francia or Scandinavia. I can't speak for the trends outside of England with as much confidence, but the 8th century was generally a period of expanding trade that probably improved the availability of good iron across Europe.
If you want to read more, I'd start with Tylecote and Gilmour's The metallography of early ferrous edged tools and edged weapons (1986). It's technical, but does a good job explaining the science that goes into iron production and the analysis that can be done on surviving iron artifacts to understand their properties.
Another good book overview is Tylecote's History of metallurgy (it looks as though someone's uploaded a pdf of it, so you can read it online)
And Pleiner has written two encyclopedic overviews of the history of iron with good discussions of early medieval europe - they're hard to find, but you should be able to track them down if you're affiliated with a university with a good ILL department.
There are two major ways that the early medieval Germanic North acquired iron.
The first is through something called "bog iron" and this accounts for the majority of "domestic" iron production. This website has a decent run down of it, and actually I think mightyflynn linked to it too while I was writing heh. As you can guess, iron is a very valuable commodity in this world and bog-iron production is a difficult and not particularly productive means of acquiring it. There is also some evidence of mining, but on a much smaller scale.
Here is a map of major areas of iron production in Scandinavia taken from the article "Iron production in Scandinavian Archaeology" published by Lars Stenvik in The Norwegian Archaeology Review 36.2 2003, pp. 119-134. This article is quite interesting (both in terms of its theoretical analysis of archaeological investigation and its discussion of iron production) and has a wealth of endnotes which you could pursue if you can get access to it.
We have evidence of iron production and working in Western Jutland from at least the Migration period. By the Viking Age we have evidence for Iron production and refining at major trade sites such as Ribe (also in North-Western Jutland).
This leads to our second major source of iron; trade. We have evidence of iron both in bar/ingot form and as finished goods (notably Frankish swords but also smaller luxury and utilitarian items) at all of the major Scandinavian trade ports and also at the major export sites of the Frankish Empire and the Anglo-Saxon Kingdoms. Iron swords would have been a really high end luxury product, traded for things like whet-stones (a major Scandinavian product) and Amber as well as raw goods.
Compared with the more civilized south Scandinavia was not "iron rich" but it was certainly an iron based society with both active domestic and international sources used to produce weapons as well as tools and luxury goods.
There are a couple of articles that touch on Iron production and trade in Vendel and Viking Age Scandinavia in the collection Jesch, Judith, ed. The Scandinavians from the Vendel Period to the Tenth Century: An Ethnographic Perspective. Woodbridge, Suffolk: Boydell Press, 2002.
Hope that gives you a general sense of the lay of the land. I can't offer any numbers with the resources available to me right now but it should at least give you a sense of how they acquired iron.
In Scandinavia during the Viking age they sometimes got it from bogs. So instead of mining, they would look on the surface of the water for sign that iron lay beneath:
Streams carry dissolved iron from nearby mountains. In the bog, the iron is concentrated by two processes. The bog environment is acidic, with a low concentration of dissolved oxygen. In the acidic environment of the bog, a chemical reaction forms insoluble iron compounds which precipitate out. But more importantly, anaerobic bacteria (Gallionella and Leptothrix) growing under the surface of the bog concentrate the iron as part of their life processes. Their presence can be detected on the surface by the iridescent oily film they leave on the water (left), another sure sign of bog iron.
http://www.hurstwic.org/history/articles/manufacturing/text/bog_iron.htm
I'm afraid I'm no expert, but your question has been up for a little while without an answer. I'll happily remove my post if asked. I'm afraid I don't have original or academic sources to provide.
Edit: it is referenced in the Kalevala, the national epic of Finland. There is a section about the discovery of bog iron, actually.