The Medieval Period (Middle Ages) began with the fall of the Western Roman Empire in the 5th Century – and lasted until the 15th century AD. Also in the 5th Century, Celtic Christianity was brought to Ireland with Saint Patrick.

Painting from the Manesse Manuscript: Northern Switzerland Monastery - 14th Century

The Celtic Culture spanned through the Medieval ages, being established in Europe from the 7th Century BC to current day. The Romans called them Galli and the Greeks called them Keltoi-- both meaning barbarians.

Like most Celtic knowlege, the traditional music (Jigs and Reels) were passed down through the generations orally – meaning that they were passed on verbaly and not written. Unfortunately this makes it rather difficult for historians to prove details with accruacy and we need to rely on Roman or Anglosaxon documentation (with inhertent cultral bias and interpretation). 

Bang in the middle of the middle ages, in 1188, a Welsh Monk by the name of “Gerald of Wales” (Giraldus Cambrensis) wrote about Irish Celtic music, describing it in detail...

“For their modulation on these instruments, unlike that of the Britons to which I am accustomed, is not slow and harsh, but lively and rapid, while the harmony is both sweet and gay. It is astonishing that in so complex and rapid a movement of the fingers, the musical proportions can be preserved…”

So from this we have some indication that traditional Irish tunes date back to at least the 12 century, predating musical notation.

Painting: Raymond Meylan "The Flute" (1340)

In medieval times musicans would play harps, flutes, pipes, lutes and the fidel (which is the ancestor to the Violin – which is why in Celts call it a “Fiddle”).

The Medieval Fidel

Brehon Law is the body of ancient native Irish law set down on parchment from the 7th Century until the completion of the English conquest of Ireland in the early 17th century.

They were named after the wanderings lawyers, the Brehons. They show that the Irish Celts in the early medieval period to have been a hierarchical society, taking great care to define social status and the rights and duties.

One interesting law of the time...

“The harpist is the only musician who is of noble standing. Flute players, trumpeters and timpanists as well as jugglers, conjurers, and equestrians who stand on the back of horses at fairs, have no status of their own in the community, only that of the noble chieftain to whom they are attached.”

The earliest music notation we know of suceeds the Medieval period, but it may give us an indication of music of the era...

• Glory to the West - notated in the English Dancing Master, John Playford, 1651.
  
  
• Si Beag Si Mohr - Turlough O'Carolan (the blind Irish Harpist), 1670-1738
  
  
• Greensleaves - offically registered 1580. Rumoured to have been composed by Henry VIII for his lover and future queen consort Anne Boleyn.