Ooh, finally something I am qualified to comment on! As a homebrewer and someone with a B.A. in History hanging in his basement/brewery, this is something I love to talk about. I spend a lot of time brewing Anglo-Irish style beers because they're my favorite and do not travel well across the Atlantic.

Malt

So let's just get some things down for clarity's sake. Beer is fermented grain sugars. The sugars are made available by germinating barley (or other grains, but typically barley) until the acrospire is about as long as the grain. Then it's stopped by kilning. This process frees up starches and enzymes that the plant would normally use to grow. The grain is kilned dry, roasted, or steeped and kilned. These process create different types of malt: base malt, roasted malt, and caramel/crystal malt. Malt, when crushed and held at a temperature between 145ish-160ish will allow enzymes to convert starches into sugars. Unlike wine or cider, not all of these sugars are fermentable, which is why beer has more body than those beverages. There are other kinds but the first two are of interest to us. Keep in mind the process of malting is just as important as brewing: a bad maltster will create malt with not enough sugar and too much protein, making the beer weak, hazy, and prone to infection.

Early Guinness

Guinness opened just as brewing was becoming industrialized. Back in the Anglosphere, brewing was mostly done at home. Large estates had large breweries. Malt at this time was dried with a fire instead of an externaly heated rotating kiln, so most malt would have a smokey tang. You can find smoked beers in any good bottle shop now. There are modern smoked porters like Alaska that might be a nice mental escape to 18th century beer but really isn't authentic. Schlenkerla Brewery in Bamburg, Germany does all smoked beers, one would imagine their Weizen beer to be representative of the white ales of the 18th century, but this is an aside. There were several types of malt available, blown which was essentially torrefied barley, brown malt, amber malt, and pale malt. We have all these today but they're drastically different. There was also pale ale malt (again, still around but different today) which was made without contact with the smoke, but until the malting kiln was invented it was expensive and hard to find - the famous country October and March ales of the 18th century that later evolved into our modern Pale Ale, India Pale Ale, Burton Ale and derivative styles thereof.

Because pale, amber, and brown malts were cheaper, they composed the grist for common beers. Beers at the time would have been served mild (fresh) or stale (vatted) and many were blended to the customer's taste. This has led to a oft-repeated but discredited myth that porter was invented to taste like a favorite blend of these beers called "Three Threads." Guinness' early Porter would have been made with a significant brown malt grist and would have been "vatted" for a long time, developing "stale" qualities like tartness and "funkiness" from bacteria (lactobacilius, pediococcus, and acetobacter) and other strains of yeast (Brettanomyces). Porter was the first beer made on an industrial scale, aged in huge vats, and immensely popular with the working class - named after the porters on the London docks who loved it so.

19th Century

In the late 18th century the hydrometer was invented, allowing brewers to take measurements of their beer's density relative to plain water, allowing them to calculate alcoholic strength and efficiency of sugar conversion which was very important to the bean counters. In the early 19th century Graham Wheeler invented a malt kiln that would basically industrialize the process that would allow pale malt to be made on a large scale and dark, roasted barley known as black patent malt without starting a fire. It's basically the same thing as a coffee roaster. Apparently Guinness was an early adopter of both of these which probably explains why he was so successful - he was a smart, innovative businessman as well as a brewer.

It happened that pale malt has more starches than a darker malt - the darker malts had more of it's potential extract burnt off. Once brewers figured this out, 19th century porter grists then are pale malt and black patent malt. Adulterant colorings were banned - reports were that some brewers, in an attempt to cash in on the craze, were adulterating pale beers with stuff like coal tar - not good for health! So in the 19th century porter takes on a more familiar form, tasting something what like a beer drinker now would recognize. There still would be a tang from aging, however, until Louis Pasteur's work eliminated spoilage organisms from the brewhouse. Note that there are many beers, especially from the US and Belgium, that are still made with many of these bugs and they are damn fine beers - just an acquired taste.

The Beer Today

In the 20th century, the body of Guinness was lightened with flaked unmalted barley, cheaper than malt but uses at a percentage that didn't have a huge effect on flavor. The rest of the malt was enough to convert the starches in the flaked barley. They also moved to unmalted roasted barley instead of black malt for price reasons. Remember that malting process? They figured that they would just roast unmalted grain instead. This also had the advantage of avoiding the taxes on malt, as well, apparently.

Oh, and just to avoid this: stout originally meant strong, so stout porter would just be a strong porter. There is no real difference between porter and stout other than regional and historical differences. Beer style guidelines often suggest the use of black patent malt for porter and roasted barley for stout, but this is not really a thing and beers from both sides of the aisle do this. Courage's famous Imperial Russian Stout using black malt and Guinness porters using roasted barley. Ironically there are "imperial porters" today, basically using imperial to mean stout. Also ironic, the iconic Guinness Draught, a dry stout, is basically a low ABV session beer. Most American porters are stronger.

Summary

• 18th Century Guinness Porter: smokey and with sour/stale elements, some roast from the amber and brown malt.

• Early 19th Century Guinness Porter: smoke is gone, roast malt character is there, still some funkiness, would taste like something we would call porter or stout.

• Late 19th Century Guinness Porter: now essentially modern beer, no more Brett funkiness or lactic sourness, however Guinness apparently blends some small amount of reserve beer to emulate this.

• 20th Century Guinness: Brewed with more adjuncts to lower price, move from cask conditioning to nitrogenation in the mid 20th century. The bottled Guinness Extra Stout and Foreign Extra Stout are more representative of the historical style. The iconic draught is lower gravity and more sessionable.

Sources

Brewing Porters and Stouts by Terry Foster

Amber, Gold, and Black by Martyn Cornell

Martyn Cornell - The Three Threads Myth

Ron Pattinson - 1861 Guinness XX Porter

Ron Pattinson - Early 20th Century Guinness

Ron Pattinson - Mid 20th Century Guinness

Edit: cleaned up some stuff, added sources, better summary.

TL;DR: not really.

A quick note about what beer actually is: you start with cereal grains (barley) and boil them to release their sugars, then add hops in that soup for bitterness and flavour. You put yeast in the resulting bitter soup, and that yeast will turn the sugar into alcohol - beer!

Now, you specified 18th century (Guinness got their lease for St James' in 1759), and that's quite early in their history, and in the history of the style, because the 19th century was a much more defining moment for Guinness and stouts as a whole. Let's first look at a modern Guinness.

In all likelihood you're thinking of a draft Guinness, that is, served from the tap in a pub. In case you're not aware, the Co2 that usually mixed with the kegged beer is supplemented with nitrogen in the case of Guinness, which accounts for its creamy head and (part of) its velvety texture. That's of course something that 18th century pintmen wouldn't have enjoyed, so you're better off thinking about a regular beer.

Now, modern Guinness is a 4.2 ABV beer, with ~45 IBU and .040 OG / .006 FG. Its yeast is, of course, a home strain kept by the brewery, but has evolved to reach over 75% attenuation (the amount of sugar converted into alcohol, or the difference between OG and FG. On the grain side, it contains a majority of pale malt, and a solid dose of roasted, unmalted barley - that is, barley where the germination process hasn't been activated, but that has undergone a roasting process similar to coffee beans. Flaked barley tops up the bill to add body to the beer.

The result is a beer that's relatively low in alcohol (low OG, good attenuation), dry (good attenuation), with flavours of coffee (roast barley) and a thick body (flaked barley).

Now, looking at 18th century beers, you'll get a very different picture. Porters or stouts (the distinction is meaningless nowadays as it was around 1800, with brief periods in-between where it had some merit) were meant to be cheap and nutritious, to provide energy to "porters", workers tasked with carrying stuff around London town (from French "porter", to carry).

Their dark colour was caused by the type of malt they used - rough, lower-grade malt that was also roasted (kilned). But legislation regarding beer was much stricter, and unmalted, roasted barley wasn't allowed back then - while now being the key component of an Irish stout. Flaked barley wasn't used either, for the same reason.

Brewer logs also show that the beer was much stronger, with a lower attenuation. The average of London and Dublin stouts clocked just below 7%, with an OG of 1.070, and an attenuation of ~50%. The beer would have most likely resulted in being sweeter, boozier, and "softer" (or whatever you'd want to call the opposite of "dry" when tasting beer). Some acidity/sourness from the wooden vats would also be expected - notes of cherry have been mentioned in contemporary descriptions.

By my reckoning, you'd be able to get an idea of a 18th century Guinness by getting a strong northern English brown ale, left in a glass for a bit to account for carbonation differences between then and now.

Sauce: Porter!, Pattinson

Guinness Brewery logs quoted by Jeff Alworth, allaboutbeer

Designing Great Beers, Ray Daniels

Related, what about the beers of Augustiner and Stella Artois, since they are two of the oldest breweries still in operation?