This may not throw any light on the origin of the song, but it may be useful to somebody:

This song is sung by a character called Dribbler in the play, Amphibians by Billy Roche (New York: Dramatists Play Service, Inc., 2003), page 52:

They came up from the country.
They came up from the farms.
They came up in their thousands
When they heard the call to arms.

I wanted to be a soldier boy
To see what I could gain,
But when I put on the uniform
It was then that I became

One of the old reserves,
One of the old reserves.
Up to the Curragh I was sent.
That's the place they pay no rent;

And when the sergeant saw me,
He said I did deserve
Three pints of beer three times a day
For being one of the old reserves.

One of the old reserves,
One of the old reserves,
One of the old, one of the old,
One of the old reserves.

My first night in the cook house,
My pal he said to me:
"There's something wrong with the cook
Because he's giving us eggs for tea."

Now why he should make this rude remark
I really couldn't tell,
But when I opened up the egg,
It was then that I got the smell.
It was one of the old reserves,
One of the old reserves.

To eat this egg I did my best.
It nearly paralysed my chest.
For a cook to cook an egg like that,
A shooting he deserved,
But we let him go because we know
He was one of the old reserves.

One of the old reserves,
One of the old reserves,
One of the old, one of the old,
One of the old reserves.

One of the old reserves,
One of the old reserves,
One of the old, one of the old,
One of the old reserves.