Robert Kirk (1644-1692) was a Scottish minister who apparently was a believer in the Scottish "Siths or Fairies [called] … Sleagh Maith or the Good People" - as he characterized it. These traditions were and are akin to what has been documented in Ireland, but they stand slightly apart by virtue of being from Scotland. That said, I understand the core of your question, which can be restated as, "was this source an accurate description of the pre-Christian supernatural beliefs of the Gaelic fringe of Britain and Ireland?"

First, we need to consider if Kirk's essay captured belief at the time before we can contemplate what it might said about belief more than a thousand years earlier before conversion. The definitive source on this subject is Lizanne Henderson and Edward J. Cowan's "Scottish Fair Belief: A History" (2001). At the risk of simplifying this masterwork, it is fair to say that everyone looks to Kirk as a reliable source to understand seventeenth-century folk tradition. He apparently was a believer, so at the very least, he depicted his own beliefs - in some ways as a folklorist collecting from himself, but also from neighbors who also believed. That said, we must understand that tradition is fluid, free to change over time and space, so whatever Kirk managed to capture, even in the seventeenth century, one might have been able to find beliefs and traditions that were different if collecting in another place - or even from one Kirk's neighbors. Folklore is difficult to pin down and there can be no definitive portrait of tradition at any given time and place. It's rather like photographing a rainbow from one place and asserting that this is exactly how the rainbow (or perhaps better - all rainbows) appeared when photographs taken moments later or from other places contradict that depiction in subtle ways. Folklore is fluid. Kirk is an excellent source largely because few went as far as he did at that early time to depict contemporary folklore, but we need to put him in perspective.

So then to the core of your question: can we use Kirk - or any other pre-modern sources - to project backwards more than a thousand years to attempt to understand pre-Christian belief? Many have attempted to look back in this way, but the task is not without challenges. The obstacles are enormous and in some ways deal killing. If we can accept the premise described earlier that folklore drifts from time to place in the pre-modern world, that drift is amplified by every century that passes. Kirk hints at what we might attempt to imagine pre-conversion Iron-Age folklore, but anything more than a hint opens the door to problematic speculation.

There is something to consider, however. Kirk describes the "Siths or Fairies" as supernatural beings who live in groups of families and communities. Understanding the existence of considerable variation, this tradition is held in common from Brittany to Iceland and Ireland to Sweden. Given the shared concept in this large region, we can consider the possibility - if not likelihood - that the core aspect of this tradition is shared not only geographically but also historically. How far back we can go with that idea is another matter, but it is tempting to hear something in Kirk that echoes from a distant past. To return to the rainbow, analogy, however, every time we try to pin down something in Kirk's depiction of folklore, and to carry it into the distant past, that single thing tends to dissolve.