Several similar nonsense choruses exist, most beginning 'Right folla rolla too rah lie oh (or ay'. There are many that use it and similar lines as a break between verses.
I also think any grange connection, mentioned in a couple of websites, would be incidental, although it could have been taken up by any group that meets together.
There are several English songs that bear the same burden of 'let us be jolly, and join together.'
I believe that 'union' is used in this sense of join and bond ogether. I haven't found the song in any of the usual folk references; it may not be very old.

Lyr. Add: Let Union Be in All Our Hearts (2)

1. Gather, friends, and let's be jolly
Drive away all melancholy.
For, to grieve, it would be folly,
While we are together.

Chorus:
Right folla rolla, too ra lie oh (3x)
While we are together.

2. Old King Solomon, in all his glory,
Told each wife a different story,
Of the things we take delight in,
While we are together.

3. Eating and drinking are so charmin',
Piping and dancing there's no harm in.
All these things we take delight in,
While we are together.

Cease your quarreling and fightin',
Evil-thinking and back-bitin',
All these things we take delight in,
While we are together.

Grab the bottle as it passes,
Do not fail to fill your glasses,
Toast each moment as it passes,
While we are together.

Not too different from the version in the DT. One of the websites suggesting a Grange connection says the first two lines of the original chorus were:
'Let union be in all our farms
Let all our farms be joined as one.'
No reference is provided.

The version I reproduced above from: http://www.uptownoncalhoun.org/songs/let_union_be.htm
I would guess that it is a fairly recent version.