Here it is

SEAN O'DUIBHIR A GHLEANNA
(Patrick Augustine Canon Sheehan)

After Aughrim's great disaster,
When the foe in sooth was master,
Twas you who first rushed in
and swam the Shannon's fearful flood,
And through Slieveloom's dark passes,
You wove your gallowglasses,
Although the hungry Saxon wolves were howling for our blood.
And as you crossed Tip'rary, You rised the Clan O'Leary,
And drove a crack before them as their horsemen onward came,
With our swords and spears we gored them,
As through flood and light we bored them,
Ah, but Sean o Duibhir an Ghleanna, we were worsted in the game.

Long, long we kept the hillside,
Our couch hard by the rill-side,
The sturdy knotted oaken bough our curtain overhead,
The summer's blaze we laughed at,
The winter snows we scoffed at,
And trusted in our long steel swords to win us daily bread.
Til the Dutchman's troops came round us,
With fire and sword they bound us,
They fired the woods and mountains til the very clouds were flame,
Yet our sharped swords cut through them
In their very hearts we hewed them,
Ah, but Sean O Duibhir an Ghleanna, we were worsted in the game.

Here's a health to your and my king,
The monarch of our liking,
And to Sarsfield underneath whose flag, we'll cast once more a chance,
For the morning dawn will wing us
Across the sea and bring us,
To take our stand and wield a brand among the sons of France,
And though we part in sorrow,
Still Sean O Duibhir an Ghleanna,
Our prayer is God save Ireland and pour blessings on her name,
May her sons be true and needed,
May they never feel as we did,
Ah, Sean o Duibhir an Ghleanna, we were worsted in the game.

Written by Patrick Augustine Canon Sheehan (d. 1913) and as sung by Kevin Mitchell. (Caution: the odd word may be wrong.)

Originally "Seán Ó Duibhir a' Ghleanna". A typical Victorian rendition despite its 20th Century provenance.
Some words may be misleading:-
"crack" = creach = a martial raid for plunder, especially cattle.
"Slieveloom" = Slieve Bloom = range of Mountains in County Tipperary.
"Clan" = Clann = strictly speaking the children of the family. The Scottish concept of "Clan" sits uncomfortably, I think, on Irish shoulders.
"our sharped(sic!) swords" = Shar-ped or sharpen'd to get the metre right. Metrically, this version - though not by any means traditional - follows the original Gaelic very well

There is a great recording of it sung by The Voice Squad on Liam O'Flynn's CD, "Out to Another Side." ^^