There's a good one in "Lady of Lough Rea", intended to signify something impossible (rather like the Scots, "The blude-red rose at Yule may bloom"):

"Ah, rose-leaf maid, 'tis easy to blame the wastrel now;
But, who was ever able for to harrow with a plough?"

Further to contributions from Tootler and Joe F, Yeats greatly admired Burns's line about

"The wan moon is setting behind the white wave,
And Time is setting with me...",

while Walter Scott said that

"Had we never lov'd sae kindly,
Had we never lov'd sae blindly,
Never met, or never parted,
We had ne'er been broken hearted"

contains the essence of a thousand love songs (or stories). Note, incidentally, that "parted" would be pronounced close to "perr-tit", with "hearted" rhyming.