How does Gulliver's voyage to the land of Houyhnhnms constitute the height of Swift's irony in Gulliver's Travels?

The voyage to
the land of the Houyhnhnms shows
the ultimate destination of the unfortunate Gulliver; through
each prior
voyage he has held steadfast to the morality and dignity of his homeland and
his
countrymen, although his ideals were challenged many times. However, when
confronted with what
seems a perfect Utopian society, one in which humans are
animals and horses are intelligent and
civilized, Gulliver's alliance breaks
down at last. He cannot argue with the rational arguments
of the Houyhnhnms
and becomes a self-loathing Yahoo.

...I

entered on a firm resolution never to return to humankind... in what I said of my
countrymen, I
extenuated their faults as much as I durst before so strict an
examiner; and upon every article
gave as favourable a turn as the matter
would bear.  For, indeed, who is there alive that will
not be swayed by his
bias and partiality to the place of his birth?
(Swift,

, gutenberg.org)

Thecomes
when
Gulliver accepts that he is most at home in an inhuman society; on his
return to England he can
barely be in the same room with other humans.
Throughout his travels, he always found the means
and the need to return to
his home life and land, both physically and mentally; at the end of
the book,
he is safely home, but trapped among creatures he abhors. Gulliver has completed
his
journeys and found the reason and purpose he sought, but at the cost of
his link to human
society.