These are underground structures the purpose of which is a cause for some debate. They are considered to be either for food storage, refuge when under attack and ritual use amongst other things. They are often associated with raths , early Christian settlements and cashels.
The building methods vary. The walls can be either drystone, orthostats or rock cut (i.e. cut straight into the bedrock). The roofs tend to be made from lintels (which can be seen very clearly at Poulawack (County Clare)) and it is quite common to find ogham stones used for this purpose (see Drumlohan Ogham Stones (County Waterford)).
As well as one or many underground passages they can contain chambers. Some definitely seemed to be defensive because evidence has been found at some indicating a door that locked from the inside.
Often the tunnels are on different levels and accessed through holes in the floor or ceiling of another tunnel called drop-holes.
Standing stones, also called menhirs or monoliths, are the most simple of megalithic monuments. They are exactly what they say, a stone that stands with one end set into the ground. Being simple in form does not make them simple to understand, for they have served several purposes over time. Some were placed to mark burials, others were probably erected to mark boundaries or travel routes, the purpose of others is uncertain, but it may well have been ritual.
Standing stones can vary enormously in size from a under 1m tall to over 4m. Some have been purposely shaped (see Stone Of Destiny (County Meath)) and some must have been chosen purely for their shape (see Ballyvatheen (County Kilkenny)). Most standing stones are dated to be from the Bronze Age, but some are clearly older, especially those associated with passage tombs such as at Knowth (County Meath) and Loughcrew - Corstown (County Meath).
Others have been re-used in later times (see Kilnasaggart (County Armagh) and Breastagh (County Mayo)), perhaps to try and capture some of the powers of the old gods or to legitamise a claim to land.
A barrow is essentially a mound of earth over one or more burials. They are more usually to be dated to the Bronze Age. There are many forms of barrow including ring, bowl, long and bell barrows.
Ring barrows are formed by digging a circular trench or fosse around a central burial, with no mound.
Bowl barrows are formed by heaping up soil over the burial(s) from a surrounding fosse, these often have an external bank too (see Ballyremon Commons (County Wicklow)).
Bell barows are simply round mounds with no fosse or external bank.
Long barrows are rare in Ireland and are more common in southwest England. Their shape is basically ovoid rather than round (see Ballynoe (County Down))
A fosse is a man made trench that is often associated with a bank. Fossse can be used to refer to a dyke, the circular trench around some barrows or the trench dug around a ring fort for defensive purposes.
A boulder or standing stone set outside a stone circle, quite often at a compass point or on a significant cosmic alignment (see Ballynoe (County Down)).