Found this, think it's based on an original French song by Edith Piaf. It was a minor hit for Mary Hopkin in 1976.
There's a link here where, if you can bear it, you can listen to an incredibly cheesy version of the tune played on some kind of organ.
http://users.openface.ca/~dstephen/ifulove.htm
If You Love Me, Really Love Me.
If the sun should tumble from the sky
If the sea should suddenly run dry
If you love me, really love me
Let it happen, I won't care
If a dream I feel should fall apart
I can still be smiling with my heart
If you love me, really love me
Let it happen, darling, I won't care
Shall I catch a shooting star?
I can bring it where you are
If you'll only say you care
I can make a mountain fall
I'll do anything at all
As long as you are there
When at last our life on earth is through
I will share eternity with you
If you love me, really love me
Let it happen, darling, I won't care
When at last our life on earth is through
I will share eternity with you
If you love me, really love me
Let it happen, darling, I won't care
Let it happen, darling, I won't care