Passage tombs are a category of Megalithic monument from the Neolithic period. They are found in most regions of Ireland but are more prevalent in the Northern half of the island. The usage period of Irish passage tombs date from c. 3750 B.C. to about 2500 B.C. About twenty clusters are recorded in Ireland, but the best known examples are found along a curved trajectory from the west coast to the east, including the centres of Carrowmore and Carrowkeel in County Sligo, and Loughcrew and the Boyne Valley in County Meath. Architectural features The term 'passage tomb' only dates back to the mid twentieth century. Before then, the monuments were called by other terms, such as "chambered cairns" "Danish mounds" or "tumuli". A number of authors of the modern era, including Alison Sheridan and Robert Hensey, have attempted to categorise passage tombs. Hensey suggests three categories; small and simple open monuments, often circular, such as found at Carrowmore, County Sligo; intermediate scale monuments with a cairn covering some form of burial chamber. An example of these would be the Loughcrew passage tombs. This category may have distinctive abstract carvings or petroglyphs in the style of the Irish passage tomb tradition, solar alignments, recumbent kerbs. Burial ritual may include both cremation and deposition of unburnt bones. The third category is exemplified by super monuments similar to Newgrange in the Boyne Valley. These are constructed on a greater scale, possess more elaborate artwork and are architecturally extravagant, incorporating techniques such as corbelling (this is also seen in the second category), long roofed passages, sillstones, and with building materials brought from distant locations. More elaborate passage tomb tradition monuments may have sub-divisions of architectural space; cruciform layouts are well known from Loughcrew, Carrowkeel and the Boyne Valley Sometimes the clusters of monuments have a focal or central monument, bigger than its 'satellites'. This pattern is seen at Knowth and at Carrowmore. There is often structure on an even larger scale associated with landscape features such as lakes, rivers and mountains observed in the layout. For instance at Carrowkeel, Sligo, a smaller cluster in the Bricklieve Mountains has a number of outlying sites such as the Pinnacle on Keash Hill, Ardloy, Heapstown and Suigh Lughaidh One feature of Irish passage tombs that distinguishes them from other monumental types of the Neolithic era is the longevity of the tradition. They appear to be in use