I'll copy the lyrics to here so that it should turn up on future searches.

UNCLE DAN MCCANN

Well I lately wandered over here to look for my Uncle Dan,
He left the town of Galway in the year of '51,
Where he got to I don't know
I've searched this country high and low,
But back to Galway I can't go,
Without my Uncle Dan.

Chorus:
So have you seen my Uncle Dan?
Typical bit of a Galwayman,
Came out to the USA in the year of '51.
If any of you neighbours living here
Have seen or heard of him anywhere,
You'd oblige me if you'd help me find
My Uncle Dan McCann.

Well let me tell you about my Uncle Dan
He thought he was a mighty man,
Swam the River Liffey twice a day
He could run three laps of Phoenix Park,
And dance and sing just like a lark,
And he wasn't afraid to go home in the dark,
My Uncle Dan McCann.

So have you seen my Uncle Dan McCann?
Typical bit of a Galwayman,
Fought throughout the Civil War in the year of '61.
He was supple and nimble in every joint,
He wasn't afraid of the bayonet point,
He belonged to the right in '69,
My Uncle Dan McCann.

Chorus

Well I found my Uncle Dan McCann,
Very good for a shantyman,
He has a seat in Congress, and the saviour of his clan.
He helps to write America's laws,
But his heart and soul is in Ireland's cause,
God help the man who opens his jaws
To my Uncle Dan McCann.

Repeat chorus

This is a transcription from the singing of Ken Hall, featured on the splendid CD as close as can be on Fellside FECD155. He says that he learned this "seemingly slight song" from the Napoleon of Irish song, Frank Harte, and that it is "starting point for anybody interested in the fascinating story of the Irish in America."

I would be intrigued to learn how old the song actually is. In spite of the dates given in the song, I would guess it to be of much more recent origin.