As I'm sure you'll know, Tally-ho! was originally the cry of the huntsman who spotted the quarry. It then became endemic as a general hunting expression of greeting among upper classes and was used as a chorus in many hunting songs, particularly though not exclusively the theatrical hunting songs full of flowery description of the 18th century. Such things have a tendency to filter down to the aspiring lower classes and not always in exactly the same form.

My slang dictionaries also give the meaning of 'tally' as living over the brush, what we would call today common-law partner, but I don't think this is connected.

I think 'Tally-I-O' was just another greeting in common usage possibly derived from 'tally-ho'.

Incidentally I can't yet find any examples of its usage in earlier hunting ballads but will keep a look out.