The singer mourns the loss of Portmore's trees which have been cut down and floated away by "the long boats from Antrim" The birds weep saying "Where will we shelter or where will we sleep?"
OBoyle: In 1761 the castle built in 1664 by Lord Conway was removed. When the estate was broken up many of its trees were sold. The text comments on the "woeful destruction of your ornament tree"; O Boyle says this refers to "the Great Oak of Portmore which was blown down in 1760.... It was fourteen yards in circumference."
See SHenry H775, p. 171, "Bonny Portrush" (1 text, 1 tune), apparently written by Henry "by request.... The first verse is parodied on the old ballad 'Bonny Portmore.'" I wouldn't call it a parody: the O Boyle lines are "If I had you [Portmore] as I had once before All the Lords in Old England would not purchase Portmore"; the Henry lines are "Were I near you now as I once was before, All the gold of old England would ne'er part us more."
Greig #32 dicusses parallels between "Bonny Portmore" and "Bonny Udny" and similar songs. While his "Bonny Portmore" is not this song (Greig's has nothing to do with trees) it begins with practically the same lines: "O bonny Portmore, ye shine where ye charm, The more I think on you it makes my heart warm" but continues "But when I look on you it makes my heart sore To think of the valiant in bonny Portmore." Greig notes the same sort of pattern in "The Boys of Kilkenny" ("Kilkenny's a pretty town, and shines where it stands, And the more I think on it the more my heart warms; Oh! If I was in Kilkenny I'd think myself at home, For 'tis there I get sweethearts, but here I get none.") and "Bonnie Paisley." - BS