When we look for sources of information about fairies we often, logically, turn to folklore and anecdotal accounts. And so we should as these are good, solid sources of information. But we do have another sources about the Irish Good Folk, and arguably an equally important source: mythology. In Irish culture these beings aren't limited to later folklore but appear throughout written mythology as well, going back to the 5th* century Echtra Condla.
In the earliest account, the aforementioned Echtra Condla, we find a story of a woman of the Otherworld who appears to Connla, son of the king. No one else can see or hear her but they can see Connla's interaction with her. She tells him that she is of the people of the fairy hills and describes the place as "an immortal land where there is no death or the sin of transgressions. We have our harvest feast without labor; peace cloaks us without strife". She then invites Connla to go with her, his father's druid intervenes, blocking her for a time, but eventually Connla does indeed go with the fairy woman, never to be seen on earth again.
We find further references to the aes sidhe, the people of the fairy hills, across Irish mythology including in some of the most well known stories like the Táin Bó Cuiligne. In that story a man of the sidhe appears to Cu Chulainn and Laeg, although again no one else can see him, and later in the story when he is gravely injured Cu Chulainn is taken into the Otherworld. We also see Cu Chulainn interacting with Otherworldly beings in the Seglige Con Culain, where he falls afoul of Otherworldly women and eventually ends up having an affair with Manannán mac Lir's wife Fand. We also find references to the Riders of the Sidhe in various places, including the Cath Maige Tuired and Oidheadha Chloinne Lir, where these mysterious riders are called on to aid different forces in battle (the Fomorians in the first story, the Tuatha De Danann in the second). This, as with several others examples, shows the complex interconnection between the Tuatha De Danann and the beings of the Otherworld. We also find references to the people of the Otherworld in some of the stories of Finn and the Fianna, showing the way that these beliefs stretch across various periods and epic tales.
The earliest references to what are now known as Leprechauns appear a thousand years ago in the stories of Fergus mac Leiti, notably the Aided Fhergusa meic Leidi. These tales describe beings that are 18 inches tall, connected to water, and who live in a complex society complete with monarchs, poets, champions, and common folk - very different from later folklore of the same beings. These stories and this evolution may be particularly important to study as it shows us the way that a belief can begin and change over periods of time.
When we think of the Good Folk I believe its important to look at all our possible sources of information, without limiting ourselves to any one genre or approach. There is such a huge range and depth of material out there and all of it works together to form a cohesive whole. Read the folklore, certainly, but also read the mythology and use both to shape your understanding of these timeless beings.
*the text is dated slightly later but several scholars suggest based on the language of the existing manuscripts that the story itself goes back to the 5th to 7th centuries.