Brennans theory is that the cairns work in a sequence, the position on the horizon of the sun is tracked from one cairn to the next leading to the astronomical events used to divide the year (solstice, equinox, cross quarter days). Cairns I, H, F and U he believes were used to 'warn' of approaching events, Cairn I 'warns' of approaching equinox (marked by Cairn T) and Cairn H 'warns' of the approaching cross-quarter (marked by Cairns L and U).

I have been tracking this for the past couple of years to see if he at least has his calculations and observations correct (since he was out at Knowth which is not accurate for the equinox as he claimed) and to get some half decent pics so at least it can be considered with good documentation. And it's always a very special way of spending a morning whether you think it was intentional or not, the effect is spectacular!