Nelson Mandela did indeed appear at a rally at Yankee Stadium, in June, 1990, only 4 months after his release from prison. There's an article here.

Lyrics below copied from another forum here.

THE LEGEND
written by Dan Hannon
recorded by Terry O'Neill

I went to see a legend just the other night
At the Yankee Stadium, underneath the lights.

I heard a man speaking after years and years in jail.
His name it was Mandela and he came to tell his tale.

The crowd they cheered him loudly, ah, but then the silence fell
As he spoke about the hard years in a South African cell.

And though he was free, his heart was feelin' the pain,
For his country and his people were still a part of that shame.

He said he hoped that we would join him and walk down freedom's path,
And these roads would be the hardest, oh, but they would be the last.

And to join our hands together so that we might be as one,
And to bring ourselves to the cause so his battle could be won.

I went home to bed that evening, went to sleep and I had a dream.
I was standing in the pouring rain, in an Irish field of green.

And all around the headstones were the martyrs of the past.
I stood there in silence, and they spoke to me at last.

"Kevin Barry is my name, and I died in a cell.
They wanted the names of my comrades, oh, but this I would not tell."

"And I am Roger Casement. I was shot in London town
For bringing German rifles to the lonely Banna Strand."

"James Connolly is my name. The working people are my life.
When they shot me down in Kilmainham Gaol with strife."

"And I am Padraic Pearse. I was shot down by the foe.
I read the proclamation at the steps of the GPO."

And the last voice that I heard says, "My name is Bobby Sands,
And it's good to hear Mandela's words a-ringin' through the land.

"I want to hear them in Belfast, in Derry and Tyrone.
Maybe then those English soldiers will know it's time to go home."

I woke up in the morning. I remembered them all.
And who would believe that they'd knocked down the Berlin Wall?

And I said "Mandela, could this really be?
Maybe now we will see Ireland reunited and free.
Maybe now we will see Ireland reunited and free."

[Sung by Terry "Cruncher" O'Neill on "Something Inside So Strong."]