https://en.m.wikipedia.org/wiki/C%C3%A9ide_Fields ... the currents in the Atlantic Ocean off of spain will bring you right to Ireland. There has already been significant study of how these currents took seeds and mediterranean plants to Ireland after the ice melted after the last ice age. In Britain the plants are the ones in northern France, Denmark and Scandinavian. I linked the field system because they're well known, however there are a myriad of long barrows, Neolithic tombs and standing stones that are older than those in Britain by centuries to a millenia. It's likely they spread from Ireland. You also have the Orkney island culture (stones of stennese, skara brae) which seems to have spread South at just a little later date. Arachaeologists are trying to figure out where these rituals started in the isles. Like most things, it was probably a combination of cultures that came together. One thing is for sure, they all are very similar to each other for thousands of years showing trade routes and a widespread belief system based on the seasonal equinoxes/solstices.