I came across this on youtube: "Irishman sings a beautiful ballad" and then started to search for it. Liam Hart on youtube and Philip O'Dea on facebook were the only other versions I could find. Here is the best I could pick out from the three versions plus I looked up goldrush towns in California and thought Mokelumne Hill is the spot. Plus if you are familiar with Dangerous Dan McGrew you may "bet your poke" or someone may "pinch your poke", its a little bag of gold generally worn around the neck for safekeeping. Also, there is at least one Fulton in Galway in the 1840s:

It was in the hungry forties when old Ireland lay in gloom
A thousand sailed from Galway Bay to find a brighter home
Among them were three comrades who were brave and strong and bold
And they (left/came) from Dinish Island (on/off) the Lettermullan shore

As the streets of Philadelphia they had heard were paved with gold
But very soon they realised that it melted with the snow
The pangs of hunger knows no bounds as it makes the young look old
So they went to California a hunting for the gold

They staked a claim upon the hill known as the Mokelumne
And started digging into it some fortunes for to (hold/hoard)
The promise to their parents that they had left at home
Was to bring it back to Dinish on the Lettermullan shore

McDonagh went to town one day to rustle up some stores
He was killed by three banditos who robbed him of his poke
John Fulton and Jack Larkin they waked him as at home
And they buried him next to his claim by the side of Mokelumne

And the mound was patted down and the three Hail Marys said
They placed a little wooden cross as a marker at his head
Jack Larkin faced unto the east and he swore a mighty oath
That blood for blood he'd have revenge or never face for home

That very day he went to town with a pistol in his belt
And he went in to the tavern where they killed his cousin Pat
He called out the banditos and when the fight was o'er
The three of them were lyin' dead upon the barroom floor

Jack took a bullet in his back and for three long weeks he lay
Then one night to Fulton says, sure I'll never see the day
But tell my folk in Dinish if you ever make it back
That I got the three banditos who killed my cousin Pat

He was buried with his cousin in a grave upon the hill
With a marker at his head and feet and the three Hail Marys said
Before the mound was patted down Jack Larkin's mighty soul
Had flown back home to Dinish on the Lettermullan shore