Solstice

They were like most of the ancients keen observers of the heavens. You don't need a clock. Simple enough observations and measurements of stars as they travel the night sky and the sun during the day will pretty quickly(well over the course of a few years) give you a "clock" accurate to the day. Indeed the first truly modern accurate chronometer clock made by John Harrison in the 1700's was regulated by observing the passage of a star between two reference points and that clock was accurate to 8 seconds a year.

Also these places nor the technology behind them jump up in a week. They evolve. There was likely generations of observations and oral recording as both a secret knowledge thing among the religious elite and for the very practical purpose of knowing when to plant and harvest crops or when migrating food animals were likely to show up. There may have been wooden structures present in the landscape that have since rotted away or were removed to make way for the permanent stone structures.

So that part is relatively easy. Whats not so easy is how well they measured and constructed something like Newgrange and others to take advantage of these solar and lunar alignments in such a bloody spectacular way. As such they are by far the worlds oldest working clocks and astronomical devices.




Sundials came much later, in the middle east, though if some experiments and observations are correct the very first one we know of may be carved into a kerbstone at Knowth near Newgrange.

If it can be somehow proven it is likely to be a sundial then it meant a bunch of boyos in Meath were ahead of the babylonians by a couple of 1000 years.