As a rousing pub song by Tanaman Dùl at Mittelalter Taberna in Brasília, Feb. 18, 2011
And as a very nice, contemplative piano solo by YouTube user machinehay
Sheet music (3 pages) from The Irish Melodies by Thomas Moore, arranged by Charles Villiers Stanford:
Let Erin remember the days of old.Moore's Footnotes quaoted in "Sing, Sweet Harp of Erin: Thomas Moore's Irish Melodies (1808)." Folkworld http://www.folkworld.de/37/e/moore.html:
Ere her faithless sons betrayed her;
When Malachi wore the collar of gold,
Which he won from her proud invader.
When her kings, with standard of green unfurled,
Led the Red-Branch Knights to danger;
Ere the emerald gem of the western world
Was set in the crown of a stranger.
On Lough Neagh's bank as the fisherman strays,
When the clear cold eve's declining,
He sees the round towers of other days
In the wave beneath him shining:
Thus shall memory often, in dreams sublime,
Catch a glimpse of the days that are over;
Thus, sighing, look thro' the waves of time
For the long-faded glories they cover.
 "This brought on an encounter between Malachi (the Monarch of Ireland in the tenth century) and the Danes, in which Malachi defeated two of their champions, whom he encountered successively, hand to hand, taking a collar of gold from the neck of one, and carrying off the sword of the other, as trophies of his victory." -- Warner's "History of Ireland," vol. i. book ix.
 "Military orders of knights were very early established in Ireland; long before the birth of Christ we find an hereditary order of Chivalry in Ulster, called Curaidhe na Craiobhe ruadh, or the Knights of the Red Branch, from their chief seat in Emania, adjoining to the palace of the Ulster kings, called Teagh na Craiobhe ruadh, or the Academy of the Red Branch; and contiguous to which was a large hospital, founded for the sick knights and soldiers, called Bronbhearg, or the House of the Sorrowful Soldier." -- O'Halloran's Introduction, etc., part 1, chap. 5.
 It was an old tradition, in the time of Giraldus, that Lough Neagh had been originally a fountain, by whose sudden overflowing the country was inundated, and a whole region, like the Atlantis of Plato, overwhelmed. He says that the fishermen, in clear weather, used to point out to strangers the tall ecclesiastical towers under the water.
Andrew Kuntz Fiddlers Companion http://www.ibiblio.org/fiddlers/REA_RED.htm#RED_FOX_:
RED FOX , THE (An Sionnac Ruad). AKA and see "Let Erin Remember the Days of Old." Irish, Air (4/4 time, "with spirit"). G Major. Standard tuning. AB. O'Neill (Music of Ireland: 1850 Melodies), 1903/1979; No. 390, pg. 68.More at "Let Erin Remember the Days of Old" w/ notation in D! (but for bagpipe) posted to Hogfiddle on March 17 (!), 2010.
T:Red Fox, The 
S:O’Neill – Music of Ireland (1903), No. 390
D2|G2 G>A B2 B>c|d2d2 c2 B>c|d3 e B2G2|A3F GFED|G2 G>A B2 Bc|d
d2 de B2G2|A4 G2||Bd|g2g2 f2 ed|e2 dB d2 BA|G2 G>A BAGB|A4 G2 Bd|g2g2 f2 ed|