Side note: the song was used in "Buffy the Vampire Slayer" as a mental trigger to make one of the characters turn evil. Every time he heard it he started killing things.

My ex I and I used to play it all the time. I think she learned it in her high school chorus. I know it's in the venerable book "Popular Music of the Olden Times" published by Chappel. Wikipedia lists a set of lyrics and an alternate set:

Early one morning,
Just as the sun was rising,
I heard a maid sing,
In the valley below.

CHORUS:
Oh, don't deceive me,
Oh, never leave me,
How could you use
A poor maiden so?

Remember the vows,
That you made to your Mary,
Remember the bow'r,
Where you vowed to be true,

Chorus

Oh Gay is the garland,
And fresh are the roses,
I've culled from the garden,
To place upon thy brow.

Chorus

Thus sang the poor maiden,
Her sorrows bewailing,
Thus sang the poor maid,
In the valley below.

Chorus

Alternate Lyrics:

Early one morning, just as the sun was rising,
I heard a young maid sing in the valley below:
"Oh don't deceive me, Oh never leave me,
How could you use a poor maiden so?

"Remember the vows that you made to me truly;
Remember how tenderly you nestled close to me.
Gay is the garland, fresh are the roses
I've culled from the garden to bind over thee.

"Here I now wander alone as I wonder
Why did you leave me to sigh and complain?
I ask of the roses, why should I be forsaken?
Why must I here in sorrow remain?

"Through yonder grove, by the spring that is running,
There you and I have so merrily played,
Kissing and courting and gently sporting,
Oh, my innocent heart you've betrayed!

"How could you slight so a pretty girl who loves you,
A pretty girl who loves you so dearly and warm?
Though love's folly is surely but a fancy,
Still it should prove to me sweeter than your scorn.

"Soon you will meet with another pretty maiden,
Some pretty maiden, you'll court her for a while;
Thus ever ranging, turning and changing,
Always seeking for a girl that is new."

Thus sang the maiden, her sorrows bewailing;
Thus sang the poor maid in the valley below:
"Oh don't deceive me, Oh never leave me,
How could you use a poor maiden so?"