The Irish Fae world includes the Tuatha De Danann, who were the "gods and not-gods" (in Irish: de agus ande) of early Ireland. The Tuatha De Danann were--and are--real people, or perhaps "beings" is a better word. They were also biologically compatible with humans. They married into the (very human) Milesians and had children when the Milesians conquered Ireland. (This history is documented in many ancient sources, including The Annals of the Four Masters, one of Ireland's earliest written histories, transcribed by monks.
The Milesians were the people who populated modern Ireland. Their surnames are the ones that start with O' and Mac. As times changed, the Irish emigrated to other countries, and dropped these letters from their name. For example, O'Baoighill became O'Boyle and then simply Boyle. Murphy was O'Murchadha and Sullivan was O'Suileabhain, and the list goes on.
If you have Irish ancestry, and your Irish ancestors lived in Ireland for at least three generations then you may have Milesian blood in your veins. This means you probably have Fae ancestry. If you could trace your heritage back far enough, you'd get to the Milesian ancestor who married one of the Tuatha de Danann, and you'd actually have the name of your faerie ancestor.
If you're a FitzGerald from the Limerick area, the process may be a lot easier. Lord Desmond, the third Earl of Desmond, married the Tuatha De Danann goddess, Aine. Their son, Gerald, fourth Earl of Desmond, is still seen riding from Lough Gur every seven years when the lake is dry.