I'll leave the discussion on what sorts of food and drink would have been served at such an establishment to the much more qualified /u/gothwalk , but I can discuss the second aspect of your question.

First off, pubs as we understand them would simply not have existed in early medieval Ireland. Firstly, the Irish economy was not monetary; the first coinage was only minted in Ireland at the beginning of the 11th century, and even then this was limited to Scandinavian settlements like Dublin, Waterford and Wexford. The principal form of "currency" in early medieval Ireland was livestock, specifically cattle, although slaves and ounces of silver or gold could also be exchanged. Needless to say, cattle doesn't make a convenient method of currency, but that wasn't really an issue for your average Irish person because mercantile forms of exchange would have been fairly rare and mostly limited to the wealthy; skilled artisans might produce goods like brooches, cloaks or swords for a fee, but the average person lived on a self-sufficient farmstead and would have been able to produce most of the goods that they needed on their own.

This was also complicated by the fact that Ireland was entirely rural. The Irish did not even start to live in villages until well after the Anglo-Norman conquest; as mentioned above, people would have lived with their families in isolated farmsteads dispersed across the countryside. So if Irish people didn't have money (in a conventional sense), rarely participated in market forms of exchange and didn't live in urban or even semi-urban societies, why would something that we might think of as a pub have even existed in the 10th century?

That's because society wasn't organized around market exchange as in our own society today, but through the establishment of contracts between the wealthy and noble and everyone else. Basically a person would rent a certain amount of cattle from a wealthy person, and would in exchange be expected to provide their patron with food, labour, goods and hospitality. Not only would the client receive cattle, the main form of currency, but they also gained legal protection from their patron and could establish other contracts with that patron as their legal suretor. Essentially, the entire point of this system was to keep the majority of the population in a subordinate position where they would feed and sustain the warrior-aristocracy, freeing them from labour. It was customary that a client was to give hospitality to their patron and his retinue of personal warriors (who were themselves aristocratic clients of the same patron) during certain times of the year, which entailed feeding them, giving them beer and providing them with shelter for the duration of their stay.

This is the context in which "pubs" in early medieval Ireland existed. They weren't capitalistic establishments where you just could walk in and "order a pint". Instead, they were a social institution where wealthy commoners could increase their legal and social status, to a standard approximating that of the lowest grade of king, by offering constant hospitality to these roving aristocratic patrons and their retinues without having to enter contracts with them. Such a person would be titled a briugu or "hospitaller" and would have been expected to provide food and drink to any person and his retinue who entered his home (the implication here being that a briugu's customers were warrior-aristocrats, and not your average joe) and to never refuse service to potential guests.

Essentially such a "pub" was merely a wealthy commoner's house which was well-supplied with food and drink. It was a satellite feature of the wider system of clientship and patronage which was as mentioned above, designed to sustain the aristocracy; unlike a regular client who provided them with goods and services in exchange for wealth and legal protection, the briugu offered hospitality and sustenance to the aristocracy to increase their own legal and social standing, gaining a status similar if not greater than that of the aristocrats themselves.

So the social history has been covered really nicely by /u/depanneur, but I can help with the question of what they actually drank.

So, beer at this point has many differences from beer as we would consider it today, and there are many oddities that you may not expect. Every place would have had its own peculiar style due to differences in ingredients, location, equipment and a whole host of different factors too numerous to list. For instance, if you were to go to a place where their primary crop was rye, rye would have been used in the mash extensively, lending it a spicy, nutty flavour that you wouldn't find if you were lucky enough to find somewhere that used wheat (wheat was very rarely used in medieval beer contrary to popular belief). With this said, the primary grain ingredient would have been barley, which was mostly cultivated and malted for this purpose. So a brief explanation of the malting process: Grain is soaked, it begins to germinate (and thus produce sugars). It is then kilned at a high temperature to roast the grain slightly, and kill the organism, this gives the sugars from the grain needed to produce beer. At this point, coke furnaces were not a thing, and wouldn't be introduced until the 17th century in England. This affects the grain part of beer in two major ways: First, malted barley at this point would have been kilned using a wood or peat fire, producing a grain not unlike the stuff that goes into a really smoky scotch. Second, the grain will be a darker roast, leading to a darker beer. So the baseline of what you're looking at visually, is a dark beer, that smells like a bonfire (when doing reconstructions at the brewery I worked at, people have wondered if I've set something on fire during the brewing process). Next, is a question of adjuncts. Honey was used, but certainly not for the everyday sort of beer you'd get in a place like this, unless maybe you'd stumbled (maybe literally) in on a feast day where they've brewed one of their stronger beers for the celebration. One of the things you might have encountered though, is peas or beans also thrown into the grain mash to supplement a meager harvest. In certain parts of the world you might have found fruit put in too, which has the advantage of being both sugary and flavourful whilst also kick starting the fermentation, as I'll get to a little later.

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So now, hops, or lack thereof. The earliest mention of hops in brewing is from the 12th century from Hildegard of Bingen, a German Abbess. However, it is probably safe to say they were used in beer before this date in Europe. However, at this time in the British isles, the herbal mixture used to flavour beer would have been a concoction of herbs, which as before, would vary depending on where you were. If you lived near moors, you might get a beer brewed with heather tips, which could be a risky proposition (heather tips are often host to a natural ergot fungus, which can cause hallucinations and ergotism if not washed off properly!). If you lived in a moister part of the country, you might get one made with Bog myrtle, a nice, sweet herbal, or Meadowsweet, which has kind of a sweet, grassy aroma and taste. Another common one was Yarrow, which provides a bitter, astringent flavour. You also may have got wild hops, stinging nettles, rosemary, or wormwood (same stuff as goes into absinthe). There are others I'm missing here, but you get the idea.

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Finally, there is the question of yeast. Yeast was not understood in a scientific sense until Pasteur came along in the 1800s, so whatever you're drinking at this point would have been fermented spontaneously using wild yeast and bacteria (hereby abbreviated as WYB, I can't be arsed to write that out every time). Beer today is mostly fermented with brewer's yeast, which is a domesticated yeast that gives a consistent, sweet flavour to beer, with little to no acid production. Think of the flavour difference between beer fermented these two ways here as a regular bread loaf, and a handmade sourdough. WYB on the other hand is a wildly variable mixture of Brettanomyces (the true wild yeast), acetobacter (produces acetic acid, or vinegar) , lactobacillus (produces lactic acid) , pediococcus (more lactic acid) occasionally candida and a whole host of other gribblies. This produces a beer with a noticeably sour tinge, and if left to mature, will result in a tooth strippingly sour beer with a lot of funky off flavours that nobody will want to touch, so it would have been drunk "young" to offset this, probably around 6-7 days after brewing, before it gets too sour and stanky (often seen in period sources as "stale" beer). Fruit may well have been added at this stage, as it carries wild yeast on the skin, so it provides a more reliable means to introduce the WYB into the beer to start the fermentation, if you've ever tried to make sourdough you'll know that WYB is a complete ballache to introduce to anything, even when it'd provide the ungrateful bastards with a lovely, sugary home.

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So, we now have a beer in front of us that stinks of a bonfire, maybe has pea flour added to it, and is somewhere around the colour of a modern best bitter or red ale. It probably has no head on it, and will also look cloudy (not that you can tell because it will probably arrive in a pottery mug), maybe with some sediment chunks floating around in it. You can smell some sweetness and grassiness with a bitter tinge from the herbals, with a little tinge of a yoghurty smell from the yeast. You take a draught of it. You taste the sweetness of the malt and herbals, followed by the sourness of the yeast, with a hefty aftertaste of smoke.

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I'm now thirsty and await the time when I can actually have a beer this evening.

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