As for the “when”, we start to see this development in the 12th century, a process which F.X. Martin once termed the "harsh metamorphosis". Of course, historical developments don’t simply pop out of thin air, but the 12th century is when we really start to see this appear within English (and some continental) writings. In the centuries prior to this the Irish and English occupied, as David Bethell put it “a common cultural world in which the Irish could still be teachers”. By the 12th century this was no longer the case, with the Irish being denigrated by English writers as barbarous and culturally deficient. The same can also be said of their views of the other neighbouring cultures of Wales, Scotland, even the peoples of the more northern parts of England.

The denigration of these peoples as barbarians was very much connected with the articulation of a distinctively English identity. This ‘harsh metamorphosis’ was part of the very same process by which a sense of ‘Englishness' came to be defined . Hugh M. Thomas has suggested that whereas anti-Celtic sentiment does not seem to have been particularly common in Anglo-Saxon England, it was extremely pronounced in pre-conquest Normandy. Thomas notes that ‘most important were the slurs against the Bretons and Irish’. He cites one example of a Norman poet, Garnier of Roeun, who attacked a rival Irish poet called Moriuht. In this poem Garnier attacks Moriuht and the Irish people in general as backwards pagans, devoid of proper agriculture and so on. These stereotypes would be taken up and more fully developed later on by English writers.

Identities are nebulous things and it is easier to define yourself against “the Other”, by virtue of what you aren’t rather than by what you are. To such writers being English meant being civilised and ‘civilisation’ was rather conveniently defined as consisting of all the features of English lowland life in this period. All the things which English writers describe as the essence of civility were in reality just the normal features of economic activity in England and the anglicised parts of the British Isles, ie. a well-populated landscape, with a settled society, wealthy towns and nucleated villages, a manorial economy, a cereal-based agriculture, and a well differentiated social structure with a numerous and vigorous gentry.

To these writers, being civilised meant being English to the extent that the two were essentially synonyms. The peoples of the ‘Celtic Fringe’, including Ireland - who naturally did not exhibit the same forms of economic activity given the differences in landscape, economy, cultural traditions, and so on - where therefore defined by contrast as lazy, bestial and barbarous. To a large extent these views of English image of civility drew heavily on the aristocratic and cultural values of northern France, at least in terms of its origins. In effect to be English meant not being Irish (or indeed Welsh, or Scottish).

To give some concrete examples, this new ideology of superiority can be seen in the writings of several 12th century authors such as Geoffrey of Monmouth, William of Malmesbury, and Gerald de Barri (commonly known as Giraldus Cambrensis, or Gerald of Wales). The Gesta Regum Anglorum, written by William of Malmesbury in 1125, presented a view of English history as the ‘triumph of civilisation over savagery’. To do so William had subtly altered classical definitions of barbarism, removing the implication of paganism and stressing instead its relation to secular and material culture.

The works of Gerald of Wales, namely his Topographia Hibernica (Topography of Ireland) and Expugnatio Hibernica (Conquest of Ireland), provide some of the fullest expositions of these themes. For Gerald, the Irish were, as Sarah McKibben has paraphrased it:

‘animalistic in their passions; sinful and ignorant in their irreligiosity; deficient in proper technological advancement, husbandry, and industry; and lacking in proper human cultivation and social relations’

In Gerald's view the savagery of the Irish was in large part due to their remoteness. While he considered that ‘all their habits are barbarism’, he also argued that habits are formed by ‘mutual intercourse’. The Irish had not had the opportunity to learn civilised habits because they ‘inhabited a country so remote from the rest of the world…thus secluded from civilised nations’. As a result, they learned and practiced nothing but barbarism.

From this we begin to see another aspect of the “why”. It is, of course, no coincidence that Gerald was writing these works on Ireland at the same time as many of his relatives were taking part in the burgeoning military conquest. In depicting the Irish in this way Gerald justified the English conquest of Ireland on the basis that it would be a supposed civilising mission. He also does have a number of pseudo-historical arguments for England’s right to rule Ireland. But by dehumanising the Irish as barbarous the English were also able to self-consciously assert their right of conquest and their right to dominate their neighbours.

As noted this really begins in the 12th century and although there was some degree of change over time, these views of the Irish remained remarkably consistent over the centuries. Andrew Hadfield has described Gerald as the ‘the ‘ghost in the machine’ in later English representations of Ireland for this very reason. As we move into the 16th and 17th centuries, English writers continue to rehearse Gerald’s arguments without much regard for their accuracy, even while historical writing was slowly becoming much more nuanced in other respects. It is of course in these centuries that the conquest of Ireland is finally completed.

Things are somewhat complicated by the fact two different colonial classes begin to emerge at this time - the “Old English”, contemporarily known as the “English of Ireland”, who were the descendants of the Medieval settlers (in some cases partly Gaelicised), and the “New English”, newcomers who had settled in Ireland in the Tudor period and after. Naturally, the Reformation had a huge impact on concepts of identity in Ireland. The New English tended to be Protestant and even more scathing in their attitudes of the Irish, whereas the Old English were largely Catholic and tended towards reformist views of Gaelic culture. My own research focused on the process of New English identity formation in the 17th century, but I’ll resist rambling on. Happy of course to answer any follow ups if you do want to delve a bit more into this.

In essence “Anti-Irish sentiment” developed in step with growing English power and the first stirrings of English imperialism, its articulation and development over the ensuing centuries is inextricably bound up with that process. As Stephen Greenblatt once expressed it in his book Renaissance Self-Fashioning: ‘the colonial violence inflicted on the Irish is at the same time the force which fashions the identity of the English’.