Here's the lyrics from the youtube video. There's no soundtrack listing for the film in imdb, and as the recording is from 1962 and the film was 1965 I imagine it's not from the film.

Far from Al's John Ford celtic twilight stuff, I think it sounds like something that Tommy Makem himself might have written, but on a cursory look I couldn't find an author.

Mick

---

YOUNG CASSIDY

His name was Cassidy.
They called him Johnny Cassidy.
Lived in a tenament,
A Dublin Georgian tenement.
Through war and slavery
He sang a song of liberty,
He fought with fiery pen
To stir the souls of men,
Young Cassidy.

There in that tenement,
That lonely Georgian tenement
He wrote by candlelight,
He wove his dreams by candlelight.
And when the cannon roar
Had poured out death in bloody war
His thundering, golden words
Above their noise was heard,
Young Cassidy.

He saw the poverty,
The soul-destroying poverty.
He tasted bitterness,
He burned with flaming bitterness.
He shouted loud and clear,
He wanted all the world to hear
His people said he lied
Because he hurt their pride,
Young Cassidy.

And now he's gone away,
The eagle he has gone away.
The stars are dimmer now,
The Plough and stars are dimmer now.
When we found glory now
Who's going to tell our story now?
There's no more song of joy
From freedom's garden boy.
Young Cassidy.

His name was Cassidy.
They called him Johnny Cassidy.
Lived in a tenament,
A Dublin Georgian tenement.
Through war and slavery
He sang a song of liberty,
He fought with fiery pen
To stir the souls of men,
Young Cassidy.

(His name was Cassidy.
They called him Johnny Cassidy.) <repeat and fade>


Source: youtube vide of Clancy Brothers & Tommy Makem