Ha !   I found this some time ago, posted in the fiddle hang-out - 

GDAE -> FCGD = Cajun Tuning (Every string goes down)
GDAE -> GDGB = Open G Tuning (A string and E string go down)
GDAE -> GDGD = Sawmill Tuning (A string and E string go down)
GDAE -> GDAD = "Gee-Dad" (E string goes down)
GDAE -> DDAD = Dead Man's Tuning, or Open D Tuning, or Bonaparte's Retreat Tuning, or "Dee-Dad" (G string and E string go down)
GDAE -> ADAE = Old-Timey D Tuning (G string goes up)
GDAE -> AEAE = High Bass, High Counter or "High Bass, High Tenor", similar to Sawmill Tuning (G string and D string go up)
GDAE -> AEAC# = Black Mountain Rag Tuning, Calico Tuning, or Open A Tuning (G string and D string go up, E string goes down)
GDAE -> AEAD for Old Sledge, Silver Lake (G String and D Strng go up, E strng goes down)
GDAE -> EDAE for Glory in the Meeting House (G String goes down)
GDAE -> EEAE for Get up in the Cool (G string goes down, D string goes up)

I've only ever tried GDAD and DDAD - I guess the different tunings "fit nicely" ( in terms of the use of open string drones ) to certain tunes - and more specifically to certain key signatures - giving that nice fiddly, ringing, sustained pipe-like sound......

I haven't used it much at all though, I only really tried it out of interest, although I kind of get what it's all about....

Talking about "out of interest" - I also have a viola-strung fiddle - and although this is not quite as "radical" a tuning - I will often tune each string up a full tone from CGDA to DAEB - why ????   Pure convenience, suddenly it's just a full octave down from the fiddle and to put in a bass-line to tunes in G, D, A etc there is no thinking needed (apart from playing on a lower string).... but that's different.

As far as "mapping it to sheet" - you might (depending on the melody line) have to break ( if you learned like this ) the "apparent" one-to-one relationship between the note position on the page and the note position on the neck.  The notes you want are all there  - use your ear and knowledge of the scale steps / note intervals ( TTSTTTS etc ).   Besides, for the couple of simple tunes I tried, the melody was largely on the "normally tuned strings" anyway..... so there was in fact no deviation from sheet and the re-tuned strings were just used as drones... (which were not scored anyway, and just left to the player to figure out!)

Well - correct or not - having tried this, these were just my findings / feelings on the matter - it was a lot of fun trying it....  Oh - and check out Vi Wickam's Bonaparte's Retreat -  - in his comments he says he had initially worked it out in GDGD, but ended up going to DDAD

EDIT : I meant to refer to this by Ian Walsh as well - he goes through the retunig process from GDAE to DDAD... 

That - @Kiara - was one awesome piece of fiddling !  Thanks for sharing it !

Schaick said
.....   I really do need a second fiddle!!  

  Errrr.... well, of course, goes without saying... then the 3rd, and the 4th... LOLOL !!!!

  Hahahahaha : EDIT - ROFL I'm SO sorry - of course, fiddle number 5, the 5th, will of course be perfect...     I am SO SO sorry about that little musical joke - just couldn't resist...