You will usually find any bad sentiment towards any peoples comes from ancient atrocity propaganda campaigns, designed to justify invasions.
Atrocity propaganda is the spreading of information about the crimes committed by an enemy, which can be factual, but often includes or features deliberate fabrications or exaggerations. This can involve photographs, videos, illustrations, interviews, and other forms of information presentation or reporting.
The inherently violent nature of war means that exaggeration and invention of atrocities often becomes the main staple of propaganda.1 Patriotism is often not enough to make people hate the enemy, and propaganda is also necessary.2 "So great are the psychological resistances to war in modern nations", wrote Harold Lasswell, "that every war must appear to be a war of defense against a menacing, murderous aggressor. There must be no ambiguity about who [sic] the public is to hate."3 Human testimony is deemed unreliable even in ordinary circumstances, but in wartime, it can be further muddled by bias, sentiment, and misguided patriotism, becoming of no value whatsoever in establishing the truth.
And Hibernophobia can be traced all the way back to Greek geographer Strabo.
except that its inhabitants are more savage than the Britons, since they are man-eaters as well as heavy eaters, and since, further, they count it an honorable thing, when their fathers die, to devour them, and openly to have intercourse, not only with the other women, but also with their mothers and sisters; but I am saying this only with the understanding that I have no trustworthy witnesses for it; and yet, as for the matter of man-eating, that is said to be a custom of the Scythians also, and, in cases of necessity forced by sieges, the Celti, the Iberians, and several other peoples are said to have practiced it."
In 1155, the Pope had to justify giving Henry II permission to invade Ireland, and he did so by calling the Irish rude and barbaric, filthy people.
History of Hibernophobia
In 1155, Pope Adrian IV issued the papal bull called Laudabiliter, that gave Henry permission to conquer Ireland as a means of strengthening the Papacy's control over the Irish Church. Pope Adrian called the Irish a "rude and barbarous" nation. Thus, the Norman invasion of Ireland began in 1169 with the backing of the Papacy. Pope Alexander III, who was Pope at the time of the invasion, ratified the Laudabiliter and gave Henry dominion over Ireland. He likewise called the Irish a "barbarous nation" with "filthy practices".
According to Gerald of Wales
They use their fields mostly for pasture. Little is cultivated and even less is sown. The problem here is not the quality of the soil but rather the lack of industry on the part of those who should cultivate it. This laziness means that the different types of minerals with which hidden veins of the earth are full are neither mined nor exploited in any way. They do not devote themselves to the manufacture of flax or wool, nor to the practice of any mechanical or mercantile act. Dedicated only to leisure and laziness, this is a truly barbarous people. They depend on animals for their livelihood and they live like animals.
According to William of Malmesbury and William of Newburgh.
"This is a filthy people, wallowing in vice. They indulge in incest, for example in marrying – or rather debauching – the wives of their dead brothers". Even earlier than this Archbishop Anselm accused the Irish of wife swapping, "exchanging their wives as freely as other men exchange their horses".
Sir Henry Sidney
in the words of Sir Henry Sidney, twice Lord Deputy of Ireland during the reign of Elizabeth I, and in those of Edmund Tremayne, his secretary. In Tremayne's view the Irish "commit whoredom, hold no wedlock, ravish, steal and commit all abomination without scruple of conscience".
poet Edmund Spenser
"Great force must be the instrument but famine must be the means, for till Ireland be famished it cannot be subdued.
You will notice, it was once considered ok for English poets to write poems about how the Irish should be starved to death.
In 1317, one Irish chronicler opined that it was just as easy for an Englishman to kill an Irishman or English woman to kill an Irish woman as he/she would a dog.
"No Irish need apply"
No Irish need apply
After 1860, many Irish sang songs about signs and notices reading Help wanted – no Irish need apply or similar. The 1862 song "No Irish Need Apply" was inspired by such signs in London. Later Irish Americans adapted the lyrics and the songs to reflect the discrimination they felt in America
However, there is actually debate as to how much anti-irish sentiment there was in the United states.
There actually was thought to be not that much, and the anti-irish sentiment was mostly from English immigrants that brought their anti-irish propaganda to the USA with them.
Historians have debated the issue of anti-Irish job discrimination in the United States. Some insist that the "No Irish need apply" (or "NINA") signs were common, but others, such as Richard J. Jensen, argue that anti-Irish job discrimination was not a significant factor in the United States, and these signs and print advertisements were posted by the limited number of early 19th-century English immigrants to the United States who shared the prejudices of their homeland.
The fact that it has died down recently, would suggest that is to do with the fact that British immigration to the USA is no longer much of a factor anymore.
And also since the good friday peace agreement, it would likely now be considered hate speech, or incitement, worthy of a jail sentence, for Brits to speak this way about the Irish. It is no longer socially acceptable.