I came across this link while doing a bit of internet searching. They claim to have totally explained Tia, but... while there are entertaining tidbits of trivia, I doubt she can ever be "totally" explained *smiles*
Tia Dalma is a character from the movie , played by Naomie Harris. She seems to be a practitioner of Vodou, and is an associate of both Captain Jack Sparrow (Johnny Depp) and Captain Barbossa (Geoffrey Rush).
Pirates of the Caribbean Character
Name:
Tia Dalma
Job:
Unknown
Portrayed by:
Naomie Harris
Ships used:
Hai Peng, Black Pearl
Weapons:
N/A
Fictional Biography
Pirates of the Caribbean: Dead Man's Chest
Tia Dalma is a voodoo priestess who traded Captain Jack Sparrow his famous compass seven years prior for a bag of gold (is told in Disney Adventures). In , Sparrow seeks Dalma's assistance for information about Davy Jones and the Dead Man's Chest. In exchange, he gives her Jack, Barbossa's cursed monkey. She later provides shelter and comfort for the surviving Black Pearl crew after the Kraken destroys the ship and takes their captain. It is Tia who proposes sailing to the World's End to rescue Jack; but first she reintroduces the reincarnated Captain Barbossa to the stunned survivors.
Pirates of the Caribbean: At World's End
In an interview by FilmFocus, Naomie Harris reveals that Tia Dalma will journey with the Black Pearl's crew to the World's End. She is, "...not really who she says she is. She is a lot more powerful."
It's hinted that Tia Dalma and Jack Sparrow were once romantically involved, based on Sparrow's apprehensiveness over seeking her assistance and his reaction to her flirtations with William Turner. Although the exact nature of their relationship is unknown, in an interview with Naomie Harris, she says that Jack Sparrow and Tia Dalma probably were lovers. However there's a possibilty that Davy Jones and Tia Dalma were also lovers, because the locket that played music while Will Turner stole the key from Davy Jones looks similar to the locket that was next to the ring that Jack stole during his and his crew's first visit.
Trivia
* Tia Dalma's name is an anagram of Dalmatia. The Dalmatian Coast was a historical haven for pirates - once known as the Roman province of Illyricum and before that the kingdom of Illyria. Illyria was a pirate kingdom - plundering Roman and Greek vessels in the Adriatic - ruled by the pirate queen, Queen Teuta.
* Tia Dalma's firefly-surrounded dwelling is a homage to a small shack seen at the beginning of the famous Disneyland ride.
* Fans have noted similarities between Dalma and the Voodoo Lady of Monkey Island series of games. They point out similarities also between their hut interior, as well as the landscape they're located (both in bogs and swamps)
* Jack the Monkey's sudden good behaviour in Tia Dalma’s house foreshadows Captain Barbossa's presence, further illustrated by Jack perching atop a pair of boots of unseen individual in the room adjacent to the protagonists during their visit.
* Tia Dalma's theme (as heard on the soundtrack) contains many motifs from Barbossa and the Haunted Pirates' theme song from the first movie, which may foreshadow her hand in his resurrection. In addition, her theme contains several parts which are played by a music box like machine, similar to Davy Jones' theme, and contains motifs (including the main motif) from his theme as well, also suggesting a connection.
* In the young Jack Sparrow books, she's mentioned to have turned a girl named Constance into a cat. It is also revealed that Jack has known her since he was a child. However, in the movies, she appears younger than Sparrow (although not as young as Will and Elizabeth), this might be further evidence to her as-of-yet-unknown powers- not aging, or simply delaying the process.
* Tia Dalma's in-depth knowledge of Davy Jone's origin, as well as her personality and her surety that it was a woman Davy Jones fell in love with, and not the sea, could indicate that she's the "changing, harsh, and untameable as the sea" woman that Davy Jones fell in love with.
* While Jack steals a ring from Dalma's hut, we see a necklace with the same shape as the music box that puts Jones back to sleep when Will is stealing the key, also the same shape as the area where the key goes in on the Dead Man's Chest may also hint a connection between Davy Jones and Tia Dalma.