Oh for God’s sake, the meter of the original song (and of Kelly’s parody) is not iambic, but trochaic! Think Hiawatha, not Hamlet!. The odd-numbered lines in both are straight trochaic tetrameter:
Déck the / hálls with / bóughs of / hól-ly,
…
’Tís the / séa-son / tó be / jól-ly,
…
Dón we / nów our / gáy ap- / pá-rel,
…
Tróll the / án-cient / yúle-tide / cá-rol,
…
And similarly:
Déck us / áll with / Bós-ton / Chár-lie,
…
Nó-rah’s / frée-zin’ / ón the / tról-ley,
…
Dón’t we / knów ar- / chá-ic / bár-rel,
…
Tról-ley / Mól-ly / dón’t love / Há-rold,
…
So, in the passage at hand, we have:
Bárk us / áll bow-/ wóws of / fólly,
…
Dón-key / Bón-ny / bráys a / cá-rol,
…
Hún-ky / Dó-ry’s / póp is / lól-ly,
…
Chól-lie’s / cól-lie / bárks at / Bár-row,
…
(Note: the “Donkey Bonny” line appears to be an allusion to the parody of The Musicians of Bremen in Uncle Pogo’s So-so Stories.)
Observe, by the way, that the rhyme scheme has changed from ARARBRBR to ARBRARBR. The third chorus will be ARARAQAQ. Observe, too, that the rhymes prove my original point without even bringing up issues of meter.
Now, the even-numbered lines are a bit more difficult. They are in what is known as Sprung Rhythm, which has long been common in nursery rhymes and folk songs, but did not appear in formal Modern-English verse before Gerard Manley Hopkins, though it could be argued to have been part of the system of Old English alliterative poetry. Sprung Rhythm is based on a one-beat foot that can contain pretty much any number of syllables. One of my favorite examples, from Charles Williams’ The Calling of Arthur, is this devastating sprung alexandrine:
In Ló- / gres the Kíng’s / Fríend / lánd- / ed, Lán- / ce-lot of Gául.
So the original song has:
…
Fá-la-la-la- / lá-la- / lá-la- / lá!
…
Fá-la-la-la- / lá-la- / lá-la- / lá!
…
Fá-la-la- / lá-la-la- / lá-la- / lá!
…
Fá-la-la-la- / lá-la- / lá-la- / lá!
and so:
…
Wál-la Wal-la, / Wásh., an’ / Ká-la-ma- / zóo!
…
Swál-ler dol-lar / cáu-li-flo-wer / ál-ley-ga- / róo!
…
Lúl-la-by / Líl-la Boy, / Lóu-is-ville / Lóu?
…
Bóo-la boola / Pén-sa-coo-la / húl-la-ba- / lóo!
and:
…
Pól-ly wol-ly / crác-ker ’n’ / tóo-da- / lóo!
…
Án-te-lope / Cán-ta-loupe, / 'lópe with / yóu!
…
Gág-gin’ on the / wá-gon, Wil-ly, / fól-ly go / thróugh!
…
Há-rum sca-rum / fíve alarm / búng-a- / lóo!