The tombs at Carrowkeel apart from Cairn G have not been aligned to anything apart from most of them pointing towards Knocknarea, the tombs at carrowmore mostly (by no means all) seem to focus on the spot of Listoghill. Baltinglass points almost north and Fourknocks points at nowhere in particular so on balance there are probably as many that dont point at things we would see as significant.

Interestingly, since the current thinking is that the tradition started in Sligo (Carrowmore/Carrowkeel), Martin Byrne from this area gave an interesting talk about the tombs here and also the caves on Keshcorran which allow sunsets to be observed from the back of the caves. He thought perhaps the passage tombs that were built in the area and then further across the country may be an attempt to create 'artificial caves' which single out a sunset/sunrise that was important to those who built the passage, after observing the sunset illuminating the rear of the caves on Keshcorran which may have given rise to ritual practises there at the time of alignment.

Seems they can be aligned with all sorts of things and nothing at all. The Tumulus de Kercado in Carnac is aligned right on the midwinter sunrise, yet just up the road at Mane Kerioned there are two dolmens aligned directly south and one to the west.
Down in the Ardeche I discovered about 10 dolmens in one valley all aligned on the same notch on the horizon.
Thing is, if they're not aligned with something astronomical or topographical, there's nothing to say that they're not aligned on whoever built them's granny's birthday, or a slighlty less frivoulous but equally impossible to ascertain event.
I think it's always worth looking to see if there's anything there, just as long as one doesn't get hung up and start twisting the data to fit the model...

I suspect that the stars and sun and moon were believed to be either deities or to have sacred properties, to the ancients. Formal observations of the stellar bodies were just beginning to occur and, no doubt, there were magical ceremonies to invoke the 'power in the planets' in mundane life. (Wishing for luck). If the festivities, sacrifices, ceremonies took place when the sun rose on the longest day, for instance, and the sun rose behind a small hill on the horizon, for instance, then that alignment would have been associated in the mind of the community with good luck, good harvest, whatever. If the idea of using the alignments in magic rituals is possible then you'd expect monuments to incorporate as many alignments as possible. It's a similar concept to a church being consecrated for a particular saint and then windows, and other bits, dedicated to other saints.