I think I've located a source for the extraneous verses (the ones about the attempted draining of the lake). It comes from Ulster Journal of Archeology, Third Series, Vol 1 (1938) pp. 112-114, in an article by T.G.F. Paterson. I located it through a JSTOR article, so I could't print it (but did a series of screen shots).

After a review of the history, the relevant note reads as follows:

"The following poem is one of a collection belonging to Rev. Douglas Scott (footnote: Vicar of Aghalee from 1905 until his death in 1918) and given to me by his widow. It is said to have been written by a certain Mr. Harbinson. It celebrates an attempt by Arthur Dobbs, agent to Lord Conway and later Governor of North Carolina, to drain Lough Beg or Portmore Lough by a windmill and buckets. The whole scheme failed, because the water returned from springs or by underground communication with Lough Neagh."

There follows 17 stanzas (!) of excreable "verse"; an example being stanza 7, which runs:

Would it not have been a pity to have drawn it dry?
Bonnie Portmore, you need no supply
It's a harbour for shipping, and bogs doth manure,
A pleasure for strangers and food for the poor.

McGonagall could not have improved on this.

The 17 stanzas include the traditional three ("Bonny Portmore, you shine where you stand/Bonny Portmore, I am sorry to say/The birds of the forest").

My suspicion (and that's all it is) is that the Rev. wrote this stuff himself. My 2c

Jon Bartlett