Ogham dates back to before the 4th century AD. It forms the basis for the first written form of the Irish language. Each symbol corresponds to a letter in the old Irish alphabet.
According to old Irish stories each letter represented a native tree as illustrated in the image below. Surviving examples of ogham writing can be found etched on standing stones and burial stones in Ireland and the ogham alphabet was used in manuscripts as late as the 16th century.
Native Irish trees
Free online Irish History source
I've picked out The coming of cuculain below, the first of a triology of books by Standish O Grady to get you started.
The coming of Cuculain by Standish O'Grady. First published 1894
The Coming of Cuculain by the Irish author, journalist and historian Standish O'Grady (1846-1928) is a modern retelling of the exploits of the legendary hero Cuculain from the Ulster cycle of ancient Gaelic mythology.
Cuculainn is a demi-god, the son of the Celtic god Lug, one of the Tuatha De Danaan and Dectera, a female warrior and the sister and charioteer of Conchobar Mac Nessa, King of Ulster. He is raised by Dectera and his mortal father Súaltam and is taught the virtues of justice and wisdom.
Setanta grows up to become a handsome and fearless youth who undergoes a ríastrad or monstrous transformation when he is enraged. He learns to control this superhuman ability by having his charioteer make him angry before he battles his enemies.
Setanta's skills on the hurling field impress his uncle King Conchor Mac Nessa. He is invited to a feast by Culain, the high smith of Ulster. However the youth arrives late to the feast only to be confronted by a monstrous hound owned by Culain.
When the guard dog attacks him, Setanta kills the beast by firing his sliotar (hurling ball) down its throat using his camán (hurley). Culain is devastated so Setanta agrees to take the dog's place until it is replaced and earns the nickname Cuculain or 'Hound of Culain.' O'Grady's book finishes when Cuculain reaches the age of 17 but there are many other legends about his adventures in the Ulster Cycle.
In other verions of the legends Cuculain has many lovers. The men of Ulster are believed to have been afraid Cuculain would steal their wives so they persuade him to marry Emer. Emer is driven into a jealous rage when Cuculain falls in love with Fand, the wife of Manannán mac Lir, the god of the sea. Emer plans to kill Fand but is moved by the depth of their love. Both Cuculain and Emer compromise by drinking a magic potion that wipes away the memory of his infidelity.
The most well-known story of Cuculain is the Cattle Raid of Cooley. Queen Maeve, the Queen of Connaught, launches an attack on Ulster in order to steal the Brown Bull of Cooley however Cuculain defeats the men of Connaught single handedly. It is during the war that he is forced to kill his friend Ferdiad. Later Cuculain is tricked into eating dog meat which weakens his power and makes him vulnerable. An enemy fashions three magical spears to kill his charioteer, his horse and finally to mortally wound Cuculain. Before he dies, Cuculain lashes himself to a standing stone and his enemies only approach him when a raven lands on his shoulder.