First of all, we must remember than unlike today, "ale" and "beer" were not the same type of drink in early medieval times.

"Beer" or björr might refer to ale-type beer, but also to mead or even cider. Its not exactly clear if this is supposed to be a catch-all term for fermented drinks, or just misstranslation of the sagas.

the viking öl or ale, was made of spruced and malted barley. The brew was flitered crudely through a mesh of branches (probably juniper, to add to the flavor). The yeast used to make ale were cultivated and grandfathered from one batch to the other continuously.

The resulting brew was syrupy, mostly unfiltered, not pasteurised, and would have a yeast-ish taste, only countered by the taste of juniper berries and herbs added to it (hops might also be used, but were not the primary flavor as they are today).

The addition of juniper and other herbs would also have minor anti-bacterial properties, and thus extend the time ale could be stored.

Ale was most likely only weakly alcoholic, but would have a high caloric content, almost counting as a meal by itself. Because of this, ale was drank by all classes and children as well.

Ale was a very important part of viking culture and features prominently in viking mythology (the heavenly goat Heidrun produces ale/beer/mead instead of milk) as well as a staple drink of most festivities.

Ann Hagen, . Anglo-Saxon Food and Drink: Production and Distribution.

Clive La Pensee, The Historical Companion to House-Brewing

Elsa Roesdahl, Viking Age Denmark.