A remarkable resemblance has been noticed between the pagan military architecture of Ireland, and the early Pelasgian monuments in Greece. They consist of enclosures, generally circular, of massive clay walls, built of small loose stones, from six to sixteen feet thick. These forts or fortresses are usually entered by a narrow doorway, wider at the bottom than at the top, and are of Cyclopean architecture. Indeed, some of the remains in Ireland can only be compared to the pyramids of Egypt, so massive are the blocks of stone used in their construction. As this stone is frequently of a kind not to be found in the immediate neighbourhood, the means used for their transportation are as much a matter of surprise and conjecture, as those by which they were placed in the position in which they are found. The most remarkable of these forts may still be seen in the Isles of Arran, on the west coast of Galway; there are others in Donegal, Mayo, and in Kerry. Some of these erections have chambers in their massive walls, and in others stairs are found round the interior of the wall; these lead to narrow platforms, varying from eight to forty-three feet in length, on which the warriors or defenders stood. The fort of Dunmohr, in the middle island of Arran, is supposed to be at least 2,000 years old. Besides these forts, there was the private house, a stone habitation, called a _clochann_, in which an individual or family resided; the large circular dome-roofed buildings, in which probably a community lived; and the rath, intrenched and stockaded.
But stone was not the only material used for places of defence or domestic dwellings; the most curious and interesting of ancient Irish habitations is the _crannoge_, a name whose precise etymology is uncertain, though there is little doubt that it refers in some way to the peculiar nature of the structure.
The crannoges were formed on small islets or shallows of clay or marl in the centre of a lake, which were probably dry in summer, but submerged in winter. These little islands, or mounds, were used as a foundation for this singular habitation. Piles of wood, or heaps of stone and bones driven into or heaped on the soil, formed the support of the crannoge. They were used as places of retreat or concealment, and are usually found near the ruins of such old forts or castles as are in the vicinity of lakes or marshes. Sometimes they are connected with the mainland by a causeway, but usually there is no appearance of any; and a small canoe has been, with but very few exceptions, discovered in or near each crannoge.