In the same way as the 'civilised' peoples (who often got their iron from the barbarians themselves). Iron mining in the Alps had been quite common since about 800 BC (the start of the Iron Age), though the bog iron sources from Northern European marshes only started to be exploited around 500 BC. The process of retrieving iron ore from rock is not that different from obtaining copper and tin ores during the Bronze Age (though the process of extracting the iron from the ore is). We have some early iron production sites from Austria.

The extraction of bog iron is a bit different, and usually involved just dredging up nodules of the stuff from swamps, and purifying it after. This process has been the same for over a thousand years, into the medieval period, though the quality of iron obtained seems to have been lower than that of mined ores.

Note that iron was also a by-product of copper mining during the bronze age, as rocks that contain copper often also contain small amounts of iron. This iron was left behind (together with a number of other elements) in the crucibles used for copper smelting, and was used for small tools where a greater hardness than bronze is required, particularly in the application of decoration on bronzes themselves. The iron thus obtained is of very low purity and useless for larger instruments, though.