Probably the largest of the communities in Eriador is also the most familiar to any fan of Middle Earth: the Shire. Friendly, idyllic home of the friendly Hobbits. The Hobbits not having an army we have no military figures that we could use as a basis to estimate the total population, as I did for Gondor and Rohan. What we do have is the size of the Shire, some idea about roughly how densely it was settled, and various contextual references to the numbers of Hobbits. We have numerous references to groups of "hundreds" of Hobbits: at Bilbo's party, with the implication this was a sizeable part of the local population; and the hundreds of hobbits who quickly rallied to fight the Ruffians in the 'Scouring of the Shire'. We even have a few references to thousands of Hobbits, as in the following passage about Sam's work to repair the Shire - "Hobbits can work like bees when in the mood and the need comes on them. Now there were thousands of willing hands of all ages." We also have the seeming description of the Shire as a place nowhere densely packed with Hobbits and the seeming lack of any major towns or centres of population. All these point to a relatively low figure for the population of the Shire. If 'thousands of willing hands' constituted 10% of the population, as we can perhaps imagine such a popular effort doing, then that would mean a population of 10,000's, perhaps anywhere from 30,000 - 100,000.
There are other clues that point to much larger number of Hobbits. Firstly there is the size of the Shire, a country of 20,000 square miles, about 35% of the size of England. Even taking the English population as far back as Saxon times as a model, that would indicate a population of hundreds of thousands of Hobbits. This seems to conflict with the small numbers of Hobbits described elsewhere, and also the level of political and legal organisation of the Hobbits, who seem to exist with almost zero government of any kind at all, most unlike the larger populations of Rohan and Gondor who have recognisable government structures. The other fact that points to a larger population is the sheer history of the Hobbits. They have been in the Shire for 1400 years by LotR. And although there have been wars and plagues in that time, compared to Europe in the last thousand years they have led a sheltered and peaceful existence in a temperate and fertile land. To be honest their population should have grown massively in that time, which any figure below the hundreds of thousands just does not seem to match up with. 
One partial solution may come from assuming that although overall the Shire was a big place, that large parts of that area were not heavily populated. From Tolkien's maps and description it does appear that population was concentrated in a central and eastern belt. Also we are given the names of at most a few dozen settlements. Even assuming as many more un-named ones we have at most 50-100 settlements with some population scattered in between. These would have been small by any modern standard. The largest towns of Buckland and Michel Delving would not have numbered more than a few thousand, or they would have required larger scale infrastructure than the Hobbits seemed to have anywhere. Most villages would have numbered no more than hundreds. If we assume that the Shire population was spread across about half of the actual shire area, with large areas of the North-farthing, west-farthing, marish, hills, and other areas almost uninhabited, and divide the Shire up into approximately a hundred settlements (and their hinterland) with on average 1000 population each, then we start to very roughly converge on a median figure of about 100,000 Hobbits. This is a very rough figures, between the tens of thousands suggested by some information and the hundreds of thousands suggested by other information, but it's probably the best we can ever hope to do, with a likely range of perhaps 60,000-140,000 Hobbits.
To the far west of the Shire stood Lindon, an ancient Elf country on the western edge of Middle Earth. Lindon was the last remnant of Beleriand and most of the Eldar from Beleriand dwelt there. It had stood through the 2nd and 3rd age but slowly and continually diminishing. In the 2nd age it was a powerful Kingdom under Gil-Galad and led the opposition to Sauron, with a mixed population of Noldor, Sindar and Nandor elves, mostly refugees from Beleriand. But with the passing of millenia many died in the wars or left Middle Earth to sail into the far West. Lindon waned, after the death of Gil-galad there was no King anymore, and Cirdan was merely Lord of the Havens. But according to the Tale of Years still well into the 3rd Age Lindon sent armies to fight. By the LotR its population had diminished to the point where this was no longer possible anymore. Working from my figures for the population of Beleriand it is possible to calculate a rough ballpark figure for the number of survivors of Beleriand of around 400,000, with perhaps around 250,000 leaving straightaway to return to Valinor and around 150,000 remaining in Middle Earth. We can guess the population of Lindon in the early 2nd age as roughly 150,000, which it itself in the same ballpark as a large Kingdom in Beleriand. We can also surmise that this population would've actually naturally grown in the early 2nd Age, in the peace that marked the early part of the Age. Until war broke out again in 1700 SA the population spread across Lindon, Eregion and Eriador may have grown to perhaps 200,000-300,000. But this population would have declined dramatically in the War of Sauran and the Elves, the Last Alliance, and through to the end of the 3rd age, with a population of perhaps as little as 30,000-40,000 remaining, a similar size to the Falas in the 1st Age.
Also in the Blue Mountains that gave the boundary of Eriador we are told there dwelt various groups of Dwarves. Large populations of Dwarves lived at Belegost and Nogrod during the 1st Age. These mansions were apparently devastated after the First Age during the drowning of Beleriand when the River Lune cut through the Blue Mountains, producing the geography we are familiar with in the 2nd and 3rd Age. There are a few scattered references to Belegost but not Nogrod surviving into later ages, but it never apparently amounted to much, and most of its population is recorded as emigrating to Khazadum. Various groups of Durin's folk also dwelt in the Blue Mountains throughout the 3rd Age when first Moria then the Grey Mountains then Erebor were all plundered by evil creatures. Most significantly that is where Thorin dwelt directly before the events of the Hobbit. Taking the figures from the First Age as a starting point and scaling down considerably it is probably most accurate to assume a mixed population of tens of thousands of Dwarves dwelling in various parts of the Blue Mountains. Perhaps at its height reaching the high tens of thousands shortly before the Hobbit but certainly considerably lower than that by the time of the Lord of the Rings due to a large migration to Erebor after the death of Smaug.
Spreading out now through Eriador from the Shire we get to Bree-land. Bree-land was the area around the village of Bree, which included Bree itself and the nearby villages of Staddle, Combe and Archet. Bree had a mixed population of Hobbits and Men and Tolkien makes references to Bree having "a hundred stone houses of the Big People" as well as Hobbit holes and presumably various wooden buildings. From this surprisingly low figure, and from the repeated description of Bree as a village, we can assume it did not have a large population, certainly no more than a few thousand at most. We can also safely assume that Bree was the largest settlement, and so each of the other 3 villages would be some unspecified degree smaller. All in all it would appear that Bree-land would not have a total population much more than 5,000. Certainly less than 10,000 Men and Hobbits.
This leaves us with 3 other settlements in Eriador, one well known, two barely hinted at, but all quite small. The first and most famous is Rivendell, where the House of Elrond stood. Rivendell or Imladris existed from year 1700 of the 2nd age through to the LotR. It has been depicted as of varying sizes from a large house to the size of a small densely packed village. It's importance seems to belay the small size described for it. Not just its importance to the plot either. In the 2nd age and early 3rd age it seems to take an active military role, something that doesn't seem possible for a large house. Even if we assume that a large part of Rivendell's power was in the wisdom and magic of the people who did dwell there, as well as the power of the Elven ring that Elrond wore, it still seems that the population there fluctuated. Rivendell was founded by a force led by Elrond that rescued refugees from Eregion in the 2nd age, and then came under siege from Sauron for more than a year. This group could not have numbered less than a few thousand, and this seems a reasonable guess for Rivendell's population in the 2nd Age. Certainly by the LotR it had diminished in a similar manner to Lindon and probably no more than a few hundred people dwelt there, perhaps 500 at most.
The other two settlements were never mentioned explicitly by Tolkien but can be deduced. Tolkien a number of times described an Eskimo like population living in the far north adapted to the arctic conditions there. These aided Arvedui, the last king of Arnor, but there is no reason to think they didn't survive to the time of LotR and their population probably numbered in the thousands scattered across a vast area in the far north.
The 2nd group is slightly more controversial. Logic dictates that Aragorn and his rangers must have been a part of a larger population, purely to account for their wives and children. Evidence suggests, and Tolkien confirmed this in one of his letters, that actually a larger Dunedein people still existed at the time of LotR, from which the rangers were drawn. This fits with their description as the last Nobility and Knights of the Lost Kingdom of Arnor. Simply put, if they were the last nobility, then you would except there to be a few other non-nobility as well. Tolkien states that they dwelt in the Angle near Rivendell, in the east of Eriador. This would have been a safe and hidden location close to Rivendell. We are never told how many Rangers there are. Halbarad gathers 30 Rangers in haste to travel south to fight in Gondor, which suggests the total number was much higher than that. The way the Rangers are described suggest there can't have been that many of them though, or they would have constituted a recognisable army. A figure in the hundreds is almost certainly accurate, but whether low or high hundreds is impossible to say. This would have been part of a larger Dunedein population probably numbering in the thousands, but again an exact figure is impossible to give.
