Definitely Tom Lehrer's 'Ballad of Maureen O'Blivion' which, he said, was an old Irish ballad that differed from all other old Irish ballads only because he wrote it.

About a maid I'll sing a song,
Rickety-tickety-tin,
About a maid I'll sing a song
Who did not keep her family long,
Not only did she do them wrong,
But she did every one of them in,
Them in, she did every one of them in.

One day when she had nothing to do
She sliced her baby brother in two
And served him up in an Irish stew
And invited the neighbours in.

she set her sister's hair on fire,
And as the smoke and flame rose higher,
Danced around the funeral pyre,
Playing her violin.

Her mother she could never stand,
And so a cyanide soup she planned.
Her mother died with the spoon in her hand
And her face in a hideous grin.

One morning in a fit of pique
She drowned her father in the creek.
The water tasted bad for a week,
And we had to make do with gin.

And when at last the Police came by
Her terrible tricks she would not deny.
To do so she would have had to lie ---
And lying she knew was a sin.

And so at last I end my song
(Line forgotten)
You've yourself to blame if its too long --
You should never have let me begin.

From memory, so probably not quite accurate. A double CD with it on has just been released in the Uk, so I'm sure it's easy to find.