Were the ‘legendary’ names of ancient Greece common among the population of the time? Ie: were there people named Hercules, Icarus, Midas, Narcissus, Odysseus, etc getting around?
The short answer to your question is that, yes, there would have been real people with the names of heroes and gods, but no, they were not common.
The long answer is that Ancient Greek naming conventions are a complex and fascinating topic. On the one hand, there are no surviving sources that explain to us how or why the Greeks came up with names for their children. Ancient authors do not seem to have found this topic interesting enough to write about. On the other hand, Greek names form a tremendously large body of evidence – the Lexicon of Greek Personal Names claims to have published as many as 215,000 so far – and the fact that most names are composed of words with a direct meaning in Greek means that they present a unique window into the social world of Ancient Greece. Because of their components and their meaning, names can often tell us about people’s place of origin, family ties, social status, cult practices, looks, and so on.
With the exception of some restrictions on the naming of slaves, there do not seem to have been any particular rules about what people could and could not be named. We might expect, for instance, that it was frowned upon to give your child the literal name of a god or goddess, but there’s no evidence that the Greeks actually found it taboo. Admittedly the names of divinities, while becoming more popular after the 1st century AD, are rare enough in the Classical period (5th-4th centuries BC) that individual known cases have been extensively discussed by scholars. However, they are not altogether absent. There certainly were people named Artemis and Leto. This shows that there were no hard limitations to what a child could be named, but only conventions by which most names were chosen. The only names that genuinely don’t seem to have been used at all were those of underworld gods (Persephone, Hades).
So what were the conventions they stuck to? One of the most powerful factors, especially among elite families, was the names of ancestors; some sons were named directly after their fathers (such as Perikles, son of the famous Perikles), while others were named after their grandfathers (like Kleisthenes, father of democracy, grandson of Kleisthenes the tyrant of Sikyon). Modern scholars are on pretty firm ground when they assume that people with similar names are related; often a name would “run” in just one family, sometimes for centuries. A related strong influence was the desire to express social status (again especially among the rich), which meant that many wealthy people would have names that contained words like aristo- (the best), -archos (leader), and especially mutations of the word hippos (horse). The latter is often regarded as a firm indicator of high status, since only the richest men in Greece would be able to afford to own horses, and horse-riding was the favourite pursuit of the leisure class. Famous examples include Perikles’ father Xanthippos (“yellow horse”), the physician Hippokrates (“horse power”), and Philippos II of Macedon (“horse lover”).
If there were no particular traditions binding new parents, they would be able to choose a name they liked. Endless possibilities are known. Particularly striking to us, though not necessarily the most common, are male and female names that contain a direct reference to warfare – Kallimachos (“beautiful battle”), Archestrate (“army-leader”), Nikomachos (“victory in battle”), Andromache (“battle of men”), Deinomache (“terrible in battle”). Other names are more straightforward, like Leon (“lion”), Kephalon (“head”) or Melissa (“honey”). People were named after cities, mountains, and, very commonly, rivers; Aristotle complains about rare and ridiculous names like Hermokaikoxanthos, a pile-up of the names of 3 rivers. Generally, words like kalos (beauty), stratos (army), agora (marketplace), demos/damos (people), -anax (king) and old values like bia (strength) and kratos (power) occur often in what we might call “posh” Greek names. Names ending in -kles (Perikles, Themistokles, Damokles) refer to kleos (glory). The combinations of these words don’t always make sense; what sort of a name is Isagoras (“equal marketplace”) or Iphikrates (“strong strength”)?
Interestingly, while the actual outright names of gods are extremely rare, by far the most common type of name in Ancient Greece was actually the indirect reference to a god or goddess. There were 2 ways to do this. First, the name of a god could be easily adapted into an adjective form, so as to become a name derived from a god rather than the god’s name itself. Some of the most common were adaptations of Apollo (for example into Apollonios), Dionysos (Dionysios), Artemis (Artemisia) or Demeter (Demetrios); the latter is still in common use now, in its vowel-shifted form of Dimitri. One of the most famous examples of such a godlike name is Alexander the Great’s companion Hephaistion (from Hephaistos). The second way to adapt divine names was to add words to them – most commonly kleos (glory),-doros or -dotos (gift, given). So we have authors like Diodoros (“god-given” – the reference is to Zeus), Apollodoros, Asklepiodotos; in the Roman era, when Eastern gods made their way into Greek lands, new names like Isidoros (gift of Isis) crop up. While it has proved impossible to say with categorical certainty that names are an indication of which gods were being worshipped in a given place, regional patterns are very clear in the evidence, and names derived from gods are at least a clear indication of which divinities were considered important. Perhaps the most touching are the few cases when parents who consulted the oracle on matters related to childbirth would name their child after the god who had advised them or after the place of the oracle.
Given all these factors and trends, it’s perhaps easy to understand why the literal names of heroes were not commonly used as names, even if there were no strict rules or moral taboos against them. There wouldn’t have been any family tradition to do so; there was no social credit in making pretentious references; since mythological figures were not always the recipients of cults, they wouldn’t often have been credited for advice or protection in childbirth. It’s possible that their names would have been regarded as old-fashioned or an ill omen, given the fates of most of them. According to the searchable part of the Lexicon, Ikaros is attested just 20 times; Narkissos a more respectable 74 times. I can’t find a single person named Odysseus. Only the hero Iason has a famous “real-life” counterpart in the 4th-century Thessalian tyrant Iason of Pherai.
Source for most of this: R. Parker, ‘Theophoric names and the history of Greek religion’, in S. Hornblower/E. Matthews (eds.) Greek Personal Names: Their Value as Evidence (2000), 53-79