A 15th century ale is a rather different beverage from what we think of as beer today, though at that time it's starting to take on a few characteristics of the modern drink.

The ale your characters are drinking is probably home brewed. At this time commercial brewing is just beginning in England, though some Germans towns have been doing it for quite some time. It's probably fermented in an open container, perhaps covered with a light cloth to keep the flies (and the dog) out, and certainly not covered tight enough to add carbonation as we know it today. It would come from the brewing process with just a little carbonation from the fermentation. It would preferably be served as soon as possible, within a few days, and during that time would probably be stored and served in an open container. Once fermentation had fully ceased any carbonation would disappear quickly and the ale become "still", which was not desirable.

Should it be destined for transport (and this would be rare, as most ale would be consumed on site), it's possible it could be put in a tight container, which might give it a little bit of extra carbonation via a secondary fermentation, though almost nothing we would recognize as carbonated today. It's unclear historically if this would have been considered good or bad; bubbles in wine were certainly considered to be out of style, even in Champagne. While live beer was clearly preferred, not much is said about real carbonation. So in whatever container your characters may have contrived to carry the beer, whether a jug or skin or bottle or barrel, it might get some slight carbonation (but not enough to cause the beer to foam up on pouring.) And they may or may not find it distasteful if so.

To go a little further than your question, just so you get the full experience, what they're drinking will have a few other significant differences from your modern conception of beer. For one, you note it's an ale. Today that's not much of a distinction, but at that time in England beer and ale were recognized as two different things. Ale was the traditional drink, made (ideally, but people obviously added flavorings and adulterants) only with barley, water, and yeast. That's right, no hops. "Beer" was an import, made with hops and less strong than an ale. Hops lend a bitter taste to beer, and without them it'll be neutral to sweet. Hops are also a preservative, so you'll want to drink it in a few days before it goes bad.

Even as a strong ale, it probably wasn't very strong. Brewing techniques of the day weren't very efficient. In fact, to get all the sugars out of the grain they would make several batches of beer off the same grain, the first being a normal or a "strong" beer (depending on how much grain was used) and the later ones being "small" or low in alcohol, suitable as a refreshing and nutritious everyday drink. A strong ale would indeed be more suited to travel (as alcohol is also a preservative) but would be rather expensive. It would also not be particularly strong by today's standards, probably no more than 5% alcohol, though of course they'd have no way to measure it.

You would of course have no refrigeration, so you'd drink the beer at environmental temperature. This would bring out more of the flavors.

Modern beers are generally filtered; your 15th century beer would not be, and would taste more yeasty, more like a "liquid bread". If it was carefully brewed, it might be clear-ish (though you wouldn't have a nice clear glass to tell), but apparently quite a lot of brews were "smoky" or cloudy with yeast sediment (or other stuff.) The color would probably be a light brown, but this could vary depending on the brewer's choice of malt roast; lightly or heavily roasted malt would have been an option, but a mid-way choice seems most likely.

And if your brewer wasn't careful, or just got unlucky, a wild yeast or bacteria may have entered the fermentation. This might make for some nice fruity flavors, or make it taste a bit like paint thinner. In any case, you probably don't get the nice neutral yeast flavor we know today.

All in all, an ale in your time period would be a lot more like the modern experience of a wine, though not so strong or refined. You might get something of the taste by taking a modern barley wine, letting it sit out overnight, and watering it down by half.