A stone laid flat across an opening or passage to form a roof. This can apply to the roof of a souterrain or the top of a door or window.
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.