Well, your question is operating from a false premise. It wasn't only the northern part of Ireland that was colonised, even though the current existence of Northern Ireland today may lead you to that impression.

Attempts at English colonisation of Ireland date back to the twelfth century. Actually until the seventeenth century it was the north of Ireland, ie. Ulster, which was actually the least colonised, the most Gaelic and thus considered the wildest and most dangerous part of the country by English observers. The Plantation of Ulster in 1609 was a complex piece of social engineering which represented an attempt to change this state of affairs once and for all.

However, there were various attempts to colonise Ireland throughout the medieval period too. As noted this first began in the twelfth century, I’ll spare the detail here though. You can also read this answer - https://www.reddit.com/r/AskHistorians/comments/r7nsj7/comment/hn1r927/?utm_source=share&utm_medium=web2x&context=3 which I also borrow some of here.

English nobles were granted Irish land with a series of speculative grants from Henry II. Speculative because they comprised the territories of existing Irish kingdoms, with the kingdoms of Midhe, Ulaidh, Thomond and Desmond being granted to English barons as the lordships of Meath, Ulster, Limerick and Cork. For at least the next three decades, the conquest of Ireland was driven in large part by the efforts of these provincial lords to realise their grants. In the longer term there was naturally also a degree of intermarriage and cultural fusion between English and Irish.

These English lords also brought with them English settlers (and indeed some others, Welsh and Flemish) to work the land. The conquerors created ‘rural boroughs’, nucleated agricultural settlements which offered prospective tenants attractive conditions of tenure and the like. The Deeds of the Normans in Ireland (“La Geste des Engleis en Yrlande”), a thirteenth century Norman-French verse account of the conquest describes these land grants, writing:

“… in this manner
the country was planted
with castles and fortified towns
and keeps and strongholds
so that the noble and renowned vassals
were able to put down firm roots”

As always the reality is probably a little more muddled. Marie Therese Flanagan has pointed out that “the numbers and density of actual settlers are very difficult to estimate” and that probably less settlers than were hoped were enticed to settle on this newly colonised land. Certainly this was the case early on. Nonetheless, it was certainly an attempt at colonisation and some amount of settlers did come over. They did indeed put down firm roots. In later centuries their descendents would become known as the “Old English” in the historiography. The modern Irish nation also consists of people whose ancestors would have been these English settlers - most notably the great many “Fitz” surnames, but others too.

In theory the entirety of Ireland was claimed as a parcel of the English crown, but of course there were significant limitations to these medieval attempts at colonisation. Large areas remained under direct Gaelic control, and even in the densest areas of English settlement Gaelic populations persisted in the less arable lands - mountains, woodlands and bogland. It was precisely the limitations and ultimate failure of the medieval colonial project that formed the context and set the impetus for the policy and practice of English governance in Ireland as we move into the early modern period, along with the debates which attended this.

For a more detailed discussion you may read the answer linked above, but the sixteenth century is when we see renewed attempts at English settlement in Ireland. By this time Ireland was a patchwork of independent or semi-independent fiefdoms called lordships, dominated by a handful of Gaelic and Anglo-Irish aristocratic families. The English had left their mark on Ireland in the medieval era; it was utterly transformed by the twelfth century conquest. However, it was still largely unassimilated. Areas of direct English control had receded to an area around Dublin known as the Pale or the “four obedient shires” - due to resurgence of the native Irish lords, as well as the political power and part-Gaelicisation of some of the descendents of those medieval English settlers. Particularly the Geraldine Earls of Desmond.

Plantation was thus conceived as a renewed attempt to bring civility, stability and above all English control to Ireland. And this does not take place exclusively or even primarily in the north. Notably there were the midland plantations of Leix and Offaly (1550 and 60s) and the plantation of Munster in the aftermath of the Desmond rebellion in 1583. There are also examples of attempted ‘private’ or ‘semi-private’ plantations, whereby grants would be made to English adventurers who would undertake to colonise those regions with their own resources and at their own expense, though with some limited aid from the crown. This is particularly where Ulster comes into the picture.

As noted, Ulster remained one of the most strongly Gaelic parts of Ireland and one of the most resistant to direct English rule. Not to say there was no success in this area, it’s important to resist such hard dichotomies when dealing with Irish history. Nonetheless, it had a vibrant Gaelic culture and powerful (and proud) Gaelic lineages - most notably the O’Neill’s, but also the O’Donnell’s and the MacDonnell’s of Antrim (a branch of the Scottish MacDonald’s). It was geographically isolated and although it had been settled in the medieval period with the Earldom of Ulster, this had collapsed and ceased to have any real political relevance.

Ulster thus represented one of the biggest problems for English administrators, it's true. In the 1570s, Elizabeth I sanctioned semi-private plantation projects in east and south-east Ulster by the Devereaux Earls of Ulster and Sir Thomas Smith. Smith was an intellectual and colonial theorist and in his writings he used Roman classical models of settlement and plantation ( he also refers to Carthage and, interestingly, contemporary Venice). Other contemporary writers freely compared his private plantation attempts to those of medieval adventurers like Strongbow, who similarly conquered Ireland ‘at his own charge’.

Nonetheless, these attempts were entirely without success. Particularly the Earl of Essex’s disastrous “Enterprise” which squandered huge sums of money. In terms of government policy though, as we have seen, there was nothing unique about these developments. They were part of a wider trend within English policy in these decades, and of course there were also the medieval precedents as noted above.

However, things changed at the onset of the seventeenth century. Following the end of the Nine Years War Ireland would experience a series of even more sweeping changes. The newly pacified kingdom offered significant new opportunities for English policy makers and would-be colonisers. This was particularly so in Ulster following the so-called Flight of the Earls in 1607. Although officially pardoned and restored to their lands after the Nine Years War, Hugh O’Neill and the other prominent Gaelic lords in Ulster fled Ireland for the continent due to fear of arrest, vacating their lands in the process. Padraig Lenihan has suggested that this was perhaps the ‘defining catastrophe of early modern Ireland’ in that it opened up Ireland - and Ulster in particular - for what was to come. What changed was the English (now British) state's capacity to carry out colonial schemes on a scale that was previously not possible.

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