When visiting Ireland you might get confused - what exactly is the difference between a wedge tomb and a passage tomb? What is a rath? And when exactly is an island a crannog? And where do the Fianna and the fairies fit in?
Let me help you out with some basic explanations, sorted by alphabet:
Roughly said a cairn is an artificially constructed heap of stones. Queen Maeve's Grave on top of Knocknarea (near Sligo) is a prime example. Here we actually do not know whether the cairn is solid or a tomb.
Cashels are basically ringforts built mainly of stone. Often this takes the form of an earthen enclosure with and outer ditch and an inner earth-wall, topped by an additional stone-wall. The latter could be either a basic breast-high structure or a massive construction.
First appearing around 3,500 BC these are (usually) half-moon shaped tombs with a pronounced "courtyard" in front of the entrance. The courtyard was supposedly used for rituals, either during burials or at festive occasions.
Crannógs are ringforts on small islands near a shore - the fort is identical in size to the island, both are often connected to the mainland by a narrow bridge or causeway. The island could be either natural or artificially created (or expanded). As a rule the more circular an island the more likely it is to be artificial.
Dolmens are the uncovered remains of portal tombs. The most famous Irish dolmen is Poulnabrone in the Burren.
Generally anything that cannot be identified and encloses a part of the landscape is referred to as an enclosure - descriptive but not very definite. What this tells you is that there is a man-made structure we do not know a lot about. It could be ceremonial or military, a ringfort - the main difference being that military structures tend to have a ditch outside the walls for practical reasons. Enclosures might also be found in conjunction with tombs and/or henges. Navan Fort (near Armagh) seems to have been a ceremonial enclosure, so were some earthworks on the Hill of Tara.
After a few millennia of existence the passage tombs and similar buildings were re-interpreted as gates to the otherworld and dwelling places of fairies. This may in part be a reflection of the mysterious symbols carved into the stones and artifacts that could be found in or near tombs.
Henges are circles built out of stones or wood, they have a purely ceremonial background and might have astronomical or geographical alignments. None of the Irish henges are as spectacular as Stonehenge in England.
Heroes' Graves and Beds
Partly destroyed and uncovered tombs, open chambers and dolmens were often re-interpreted in the light of Celtic mythology - mostly the Fianna cycle. Ireland abounds with structures said to be the (often final) resting places of heroes and lovers.
Hill forts are either ringforts or ceremonial enclosures, located on a hilltop. Sometimes these hill forts are combined with or even placed on top of tombs.
La Tène Stones
Only found in Turoe and Castlestrange, La Tène Stones are basically standing stones with carvings identical to those of Celtic tribes on the European mainland.
"The old straight track" can be found in Ireland too - ley-hunters have identified several good examples. But as the science, history and even existence of ley-lines is disputed the field is wide open for interpretation. Basically ley-lines are alignments connecting important places, forming a grid on the landscape. As these alignments are far less supported by hard evidence than the astronomical or solar alignment of an individual site a lot of ley-hunting quickly descends into mere speculation.
Standing stones bearing inscriptions in the ancient Ogham-system, a special written language mainly used in Ireland. Unfortunately the inscriptions are generally very short and not very interesting. Ogham stones form a "bridge" between pre-historic and early Christian times.
Passage tombs are round tombs with a definitely identifiable passage leading from an entrance to the burial chamber. Most popular around 3,100 BC. One of the best-known passage tombs in the world is Newgrange, though nearby Knowth actually has two passages. Tombs like these two or the main tombs at Loughcrew often have spectacular astronomical, especially solar alignments. Geographical alignments seem obvious at Carrowmore.
Portal tombs are constructed out of three (sometimes more) massive standing stones, bearing an even more massive slab. Looking like a portal. The covering slab can be up to 100 tons in weight and forms the roof of a chamber. Most portal tombs were erected between 3,000 and 2,000 BC.
These are ringforts located on promontories, one side of the "ring" often consisting of sheer cliffs. The Aran Islands do have the most spectacular forts of this kind, especially Dun Aonghasa.
Raths are ringforts consisting mainly of a ditch and an earth wall - the last usually topped by a wooden palisade.
Any roughly circular fortification from prehistoric times is generally called a ringfort - raths, cashels, promontory forts and cashels being examples. The distinction between (defensive) ringforts and (ceremonial) enclosures is not always easy as both make use of walls and ditches. A fort will usually have the ditch outside the wall to make things harder for attacking enemies.
Souterrains are cellars, underground passages created near settlements and believed to have been used as storage areas, hiding places and escape routes. Some appear near tombs such as Dowth (near Bru na Boinne), leading to considerable confusion amongst antiquarians.
Standing stones are basically monoliths placed on their own or forming part of a henge. In conjunction with tombs, enclosures or natural features even solitary standing stones may have astronomical, solar or geographical alignments. Some standing stones were erected for purely practical purposes, though - as scratching posts for cattle.
Wedge tombs are very similar to court tombs - actually they look like truncated court tombs. Leading to the impression of a "wedge", hence the name. Popular from 2,000 BC.