Myself, I wondered if the arrival of the Celts in Ireland could have had anything to do with the construction of these tombs. But the timing is probably wrong. Maybe the people associated with the Tuatha de Danann?
There are theories based on what art is used when and possible connections with continental art, specifically Brittany and Iberia. Could there be a relationship with the Kurgan mound builders of early Indo-European civilization?
Interestingly, archaeologists believe the earliest megalithic art at Brú na Bóinne was
made by the original, Irish, neolithic inhabitants. The original art was simple, while later art was more sophisticated. In fact, sometimes later art was superimposed over original art.
The time of the construction of the passage tombs was approximately 3300-3100 BC. Archaeologists guess that the tombs continued to be used for their original purposes untill 2900 or 2800 BC.
The timber circles at Knowth and Newgrange were constructed and used in the range of 2600-2400 BC.
Have I mentioned yet that Brú na Bóinne is older than Stonehenge or the Pyramids? True, they are larger undertakings and accomplishments, but Brú na Bóinne is quite impressive for 4th millenium, neolithic Irish.
Here's an interesting paragraph on Irish prehistory:
From the Irish perspective the earliest "Irishmen" have been found to be located in the West of Ireland, and to have strong genetic links with the Iberian Peninsula, going back presumably to the beginnings of the megalithic. It is worth keeping in mind that the astronomical studies of the post-4000 BC period would be equally applicable to navigation as to agriculture. Accounts of voyages such as the Aeneid, the Odyssey, and the Argonauts have many references to star navigation -- they did not have to wait for the compass.(Speculation at this source, too, on who the emigres to Ireland were in ancient times.)
The other major tomb of Brú na Bóinne is Dowth. We did not visit, it is not in good shape, and has not been fully excavated or restored. So I won't be writing much about it.