This comes up from time to time, and it's all pretty much been said; a search of the Forum will reveal a number of answers, some less likely than others.  Wolfgang has already pointed to one detailed reference; here is a list of all the past discussions here that contain substantive information:

lyr and tune Helston Hal an Tow  Text with tune (source unspecified) and a tune of 1802.

Lyr Req: alt. verses to Hal An Tow  Includes modern translation into Cornish, links to earlier discussions and text of a modern song based on it.

Lyr Req: Hal n Toe  Discussion of historical record, the meaning of Hal An Tow, and the identity and location of Rumbelow.

Want first verse to Hal an Tow  Text, source unspecified; brief discussion of horns.

Hal An Tow: notes  A number of texts (many scarcely related) and discussion mainly on the significance of horns.

The DT file is at  HAL AN TOW.  No specific source is named, so it's probably taken from a record by a Revival performer of some sort.  The tune given, as CapriUni mentioned, is not the familiar one; since no source is acknowledged, it's hard to say whether or not it's authentic.