The irony of situation is to be found in all the four divisions of Gulliver’s Travels. In Lilliput, Gulliver finds himself in the midst of people who are no more than six inches in height. In Brobdingnag, Gulliver finds himself in the midst of people of a giant size by comparison with whom he himself is a pigmy. In Laputa and in Balnibarbi he finds himself among people who are queer in one way or another. Finally, he finds himself in the midst of the Yahoos and the Houyhnhnms, the former bearing a close physical resemblance to human beings and the latter being horses in their physical shape and appearance but having an intelligence much superior to that of human beings. In all these cases, Gulliver thinks himself to be literally among pigmies, giants, or horses as the case may be; but we realize that in each case Swift is giving us a portrayal of human beings themselves though the description of the various kinds of inhabitants of the different countries as determined by the requirements of the satirical intentions of the author. In the Lilliputians we recognize ourselves reduced to a small size. In the Brobdingnagians we recognize ourselves as seen through a magnifying glass. In the Yahoos we again recognize ourselves, with the good qualities of human beings completely left out. In the Houyhnhnms we recognize ourselves though here our good qualities are idealized and carried to perfection, while our vicious side is completely omitted.

The whole work is satirizing the political and social systems of eighteenth-century England,so lots of Irony in fact. You can check this link to read in details about the irony in situation and the verbal irony in Swift's work: http://www.scribd.com/doc/26386625/The-Use-of-Irony-in-Gulliver-s-Travels

factors that contributed to Guilliver's instance's sarcastic effect on the veracity of his travel narratives.

That is far too broad a question. The whole book is a work of satire. :)