Passage tombs are becoming widely thought of as more than just depositories for the dead. Some passage tombs such as those in Co. Sligo at Carrowmore and Carrowkeel are over 7,000 years old and its likely they had wooden precursors.
Passage tombs were designed to be impressive, and they were built in impressive locations taking many years to complete and decorate with the fine carvings such as in Knowth and Newgrange. Some locations (such as Newgrange) continued to hold special ritual meaning to later peoples who constructed kist type burials, stone circles, mounds, pits and wooden structures around the older monuments.
The passageway leads to a chamber which usually has recessed chambers in cruciform or sometimes L shaped configurations though some are simply a widening of the passage. Some were built to have special astronomical alignments which may indicate earlier structures such as wooden and stone rows or circles.
The roof of the chamber is usually a corbelled stone structure or a sinlge large slab in smaller tombs, built to weather proof the chamber. In Newgrange water flows through special channels away from the chamber and passage, this has kept the chamber dry for over 5,000 years.
Read a piece about passage tombs by Anthony Weir here