Thanks for asking. It made me for the first time read the notes to this song in P. Kennedy (Ed.), Folksongs of Britain and Ireland. Here's most of them:
"Like the Furry Dance, the Hal-an-Tow is also performed at Helston on 8th May by a procession, now of school children, into the contry to gather flowers and branches...It was originally a dance song, but the steps are lost and the custom lapsed from about a century ago till 1930 when it was revived. The oldest surviving version was published by Sandys: 1846...The meaning of the title is disputed. According to one theory it is 'heave on the rope', an adaption by Cornish sailors from the Dutch 'Haal an het touw' ('tow' is pronounced to rhyme with 'cow' in Helston today). Other think it might refer to the heel and toe dance of The Monk's March, which is still danced in the English Cotswold morris tradition.
Mordon evidently inclined to this view, for he writes that it

has every sign of being a processional morris dance even to the slow part at the beginning of the chorus...

But it seems a pity with such a Cornish-sounding title to despair of finding a link with the old language. In 1660 Nicholas Boson of Newlyn said that there the may-pole was set up by men singing 'Haile an Taw and Jolly Rumbelow'. It looks from this as though 'tow' in the seventeenth century rhymed with 'awe'...In Cornish 'Hal an to' (taw) would appear to mean 'Hoist the roof'."

Wolfgang