A traditional icelandic song was originally recorded by Bjork in 1994 for inlcusion on Chansons Des Mers Froides or Songs from the Cold Seas, an album by a French composer Hector Zazou. It tells the tale of a love tragically curtailed and the eternal hurt that results. It is also on the Possibly Maybe single CD3, the 5th single from Post, released in November 1996.
Icelandic lyrics:
Vsur Vatnsenda-Rsu

Augun mn og Augun n
fgru steina
Mitt var itt og itt var mitt
veist hva g meina
Langt er san s g hann
Sannlega frur var hann
Allt sem pra m einn mann
mest af lum bar hann
ig g trega manna mest
Mdda af tra fli
a vi hefum aldrei sst
elsku vinurinn gi

English Lyrics:
Songs of Rose from Watersend

My eyes and your eyes
Oh those beautiful stones
Mine was yours and yours was mine
You know what I mean
Long is since I saw him
Truly fair he was
All that may adorn a man
Most of the people carried him
You I long for most of all
Heavy with the flood of tears
Oh that I never had seen you
Dear beloved friend
Bjork, vocal; Guy Sigsworth, grand piano; Renault Pion, clarinet,
Lightwave, additional sound programing; Hector Zazou, electric keyboards
Songs from the Cold Seas track list:
1. Annuka Suaren Neito (featuring Vdrttina) 
2. Visur Vatnsenda-Rosu (featuring Bjork)
3. The Long Voyage (featuring Suzanne Vega & John Cale)  
4. Havet Stormar (featuring Lena Willemark)
5. Adventures In The Scandinavian Skin Trade (featuring Vimme Saari)
6. She's Like A Swallow (featuring Jane Siberry) 
7. The Lighthouse (featuring Siouxsie Sioux)
8. Oran Na Maighdean Mhara (featuring Catherine-Ann Macphee)
9. Yaisa Maneena (featuring Tokiko Kato)
10. Yakut Song (featuring Lioudmila Khandi)
11. Song Of The Water (featuring Kilabuk & Nooveya)
Possibly Maybe CD3 track list:
1. Big time Sensuality Plaid mix
2. Visur Vatnsenda-Rosu
3. Possibly Maybe live, at Wembly, September 13, 1996
4. Hyper-ballad over the edge mix
From an awesome special on the album at BjorkFanatic. This page has lots of pictures and even the chords to the song.
"At first Bjrk wanted to do the original choral arrangements and sing all the parts, but the collaboration with Guy Sigsworth wasn't good enough so they were forced to do just a vocal / harpsichord version. But the released version was, unknown to Bjrk and Guy, mixed by Hector with both the Bjrk-choral and solo versions, which simply delighted Bjrk!"
"While i was in Iceland i was told that: Vsur Vatsenda Rsu means 'song by Rsu from Vatsenda'
This woman called Rsu wrote poetry so the lyrics are from poem. The melody is an old icelandic 'lullaby'/traditional song.... "
thanks to whiskeronkitten
Here is an excerpt from an extensive article on the concept and development of Songs from the Cold Seas, found at soundonsound.com

"One of the most striking albums you're ever likely to hear is Songs From The Cold Seas, brainchild of French keyboard player, composer and sound sculptor Hector Zazou. It's a long musical voyage across the seas of the North -- the Chukchi Sea, the Greenland Sea, the North Sea, The Atlantic Ocean, the Arctic Ocean, the Barents sea, the Kara Sea, Baffin Bay, the Labrador Sea, the Sea of Okhotsk, the Berings Sea, and many others. Eleven times during this voyage, land was visited and local traditional songs and rhythms were sampled and brought back. Thus Songs From The Cold Seas contains exotic singing and percussion playing from the Ainu people of Hokkaido Island, from Eskimos in Baffin Island, from shamans and Yakuti people in Siberia, joik chanting from the Sami people of Lapland, and music from more familiar Nordic places such as Finland, Sweden, Ireland, The Hebrides, Greenland, Iceland and Newfoundland.
"Hector Zazou, well-known in France, has long been a champion of what's usually called 'World Music'. During the early '80s, he was one of the first to fuse African music with Western music styles such as rock, techno and ambient, and by the late '80s he was delving into the polyphonic vocal music of Corsica, making an award-winning album, Les Nouvelles Polyphonies Corses, that achieved cult status. And now there's his excursion to the North, musically an undiscovered country -- though this, explains Zazou, was exactly the point: "I had some propositions to work again on Mediterranean music, or on Rai, or other things, and I wasn't excited about any of them. So I decided to find something completely unknown, to work with music that hadn't been heard before in the West. The music of the South -- the Caribbean, Brazil, Africa -- has been done to death, so the only place to go was North...
     "...Slowly Zazou's collection of songs and chants grew, as did his list of contacts with musicians and singers. Some places proved harder than others: Scandinavia has a vibrant folk scene which honours the country's musical traditions, so it was relatively easy for Zazou to find good singers and interesting songs.
But Iceland, for example, proved "a complete black hole. There was nothing. I couldn't find anything. So I asked Bjork, who in turn asked her mother. Her mother sent her what I believe is the only record that exists of traditional Icelandic songs, and we recorded one of them...
    "...Zazou's main home studio tool is, apart from three ADATs, his Emulator III. Virtually all the backing sounds on
Songs From The Cold Seas were created or sampled or processed in the Emu III, with only a few exceptions, such as the keyboard sounds on Bjork's track, which were Lightwave-designed and played on a Roland JD800..."
Possibly Maybe disc 3
The instrumental version of Visur Vatnsenda-Rosu was performed as the opening song twice on the Greatest Hits tour of 2003.
1. July 5, 2003 in Lisbon Portugal.
2. July 10, 2003 at the Avrika Festival in Sweden.
You can listen to Vsur Vatnsenda-Rsu at these locations:

Songs from the Cold Seas
Possibly Maybe Disc 3
Live Box, Disc 3, Homogenic live, instrumental version, from Cambridge 2/12/98
Homogenic live CD, released June 3, 2004

You can listen to an excellent remix that replaces the clarinet with organ music, by disk 69, called the pray at church mix at sunday-in-the-park.com
Live box set individual Homogenic Live release
Who is Guy Sigsworth?
He worked on most of the songs on
Vespertine, Bjork's 4th studio album released in 2001, doing production, choral and instrumental arrangement. Sigsworth also mixed All is Full of Love on its single CD2 released in 1999. Sigsworth has also done production for Britney Spears and is part of the duo Frou Frou which released a killer dancy pop album in 2002.
Who is Hector Zazou? Below is an excerpt on a career profile from allmusic.com

In the early '80s, Zazou's began a series of very successful collaborations with Zairean singer Bony Bikaye. This was not just standard world pop, but a distinctive combination of ritualistic, tribal vocals and futuristic,percussive synth accompaniment. Zazou and Bikaye attracted the attention of European and New York City trendsetters, and their music became a fixture on the club scene for a time. But Zazou's real talent was first displayed on his next release, the bizarre Reivax Au Bongo, a so-called musical "photo-novel," with anaccompanying booklet, set in the mythical kingdom of Bongo. As composer and arranger, Zazou utilizes the vocal talents of both Bikaye and another prominent African artist, Kanda Bongo Man, as well as an operatic mezzo-soprano, but in contexts far removed from Africa, traditional opera, or anything else...And rather than sounding like a careless pastiche (which is what you'd expect), it sounds, well...like Hector Zazou. Sophisticated, charming, witty, and just a teeny bit decadent to paraphrase one contemporary reviewer, this is avant-garde, cutting-edge music that your grandmother would love (providing she was a hip grandmother).
   Having proven himself as a composer and arranger, Zazou decided to try his hand at production as well, first in 1992 with Sahara Blue, the steamy and evocative tribute to French symbolist poet Arthur Rimbaud (with Zazou also writing and arranging the music, and contributing keyboards and "electronics"), and then, two years later, radically altering his geography with the austere but equally successful
Songs from the Cold Seas, a tribute to the Arctic regions. (Again, Zazou produces, arranges, composes and contributes keyboards and electronics.) ...In 1996 and 1997, he returned to less ambitious collaborations with individual artists, including minimalist/ambient keyboardist Harold Budd and Celtic singer/composer Barbara Gogan. Lights in the Dark followed in 1998.