For those interested in ancient Irish myth and history I highly suggest purchasing "Island of the setting sun; in search of Ireland's ancient astronomers" by Anthony Murphy and Richard Moore. the Liffey press
It is utterly fascinating. I also tend to agree with them. The premise is that the ancient Irish handed down information orally, via story. If this is true, then we are left with the myriad Irish myths meaning something other than, or at least deeper than, face value religious / sacred stories. The myths were a smart method of handing down information regarding astronomical events. The invasions? The movement of constellations through the milky way - the path of the Sidhe and the illuminated Cow, and home of the Gods and ancestors. Oh, and so much more. The strong evidence that in the stone age (5000 + years) the inspiration for constructing the many sites, as far as ritual aspect, was that the path to the otherworld opened at specific moments when the sun was touching the ground, particularly at sunset. The sun, moon, and planets, as they drifted around the sky, provided the basis for the myths we have now.