Eight measures of 6/8 as an Intro Fill. Come in singing “I” on the very last beat of the 8 measure 6/8 pattern.
BB will automatically go into the first 6/8 mail loop.
I sat within a valley green,
I sat there with my true love,
My sad heart strove the two between,
The old love and the new love, - Exactly eight 6/8 measures.
The old for her, the new that made
Me think of Ireland dearly, - Four 6/8 measures plus a 3/8 measure. *** Here is the trick! ***
While soft the wind blew down the glade
And shook the golden barley. - Four 6/8 measures and we’re back into the standard jig.
There are some very interesting instrumentals and some breaks. This is just a first pass, but if you’re willing to give up some variety in the underlying rhythms there might be an easy way to deal with this that doesn’t require you to operate the pedal too much and doesn’t require you to be too precise with when you press it.
The Intro Fill should be an eight measure 6/8 pattern. No surprise there. You come in singing on the very last beat of the intro fill.
Even though you could get by with an eight measure 6/8 pattern for the first four lines, make a four measure 6/8 Main Loop pattern and let it loop twice for the first four lines. That’s the first eight measures of the song ending on “new love.”
Here comes the trick. Let the same four measure 6/8 Mail Loop pattern loop for the next two lines ending on “golden barley”, but create a 9/8 (6/8 + 3/8) Transition Fill. Start the 9/8 transition fill on the last measure of the Main Loop. If you only have one main loop, you’ll get the long measure ( 9/8 ) transition and it will “transition” back into the Main Loop. That gives you your extra “half measure” just when you trigger the transition.
You can still do something like an exact one measure break or fill for the instrumental parts and it will stay in time. If you want it to be consistent, then you should only have one fill.
You could create a much more complex song structure with more parts with more Main Loops, but you would need to be careful about triggering the long measure transition because it will actually transition you to the next section.