Well to understand modern Irish history and the roots of partition, you need to place it within a much wider context. The history of English colonialism in Ireland goes all the way back to the 12th century.

Basically the initial “English” (or Norman, Anglo-Norman, Cambro-Norman depending on your historiographical preference) incursions into Ireland established the initial colonial presence there, but it was far from a complete conquest. The English Crown nominally held all of Ireland, but in reality only directly controlled small pockets of territory. Particularly in the 13th and 14th centuries English domination began to be somewhat reversed, becoming confined to an area known as The Pale, (the area surrounding Dublin). From whence we get the modern phrase of something being “Beyond The Pale”, ie. beyond supposed English civility lived the barbarous Gaelic Irish.

The Elizabethan period then saw the first attempts to actively conquer the entirety of Ireland (or re-conquer, as it was seen from the English perspective). One key policy, first trialed in Munster in the 1580s was that of "Plantation". English and lowland Scots settlers (along with some Welsh it is often forgotten) were 'planted' as colonists in strategic areas of Ireland. Large estates where granted to these newcomers who were also encouraged to take on protestant tenants, to manage their estates in the English fashion and in accordance with English law (as opposed to Gaelic landholding customs).

The native Irish were forced to move to less arable land elsewhere, though of course they were often kept on as tenants too (economic realities being different from government wishes). In Munster the plantations collapsed in dramatic fashion by the 1580s but it did leave an important economic legacy, and up to the 19th and early 20th century there was still a substantial Protestant population im Cork for instance.

The place where these plantation schemes met with the most success was in north east Ulster. The "Ulster Plantation" or "Plantation of Ulster" is the most famous for a reason. In no small part this success was because the close proximity to Scotland meant that there was, in addition to government planted landlords, a large influx of 'unofficial' settlers welcomed across by the new British landlords to replace what they saw as unreliable Catholic Irish tenants. The Ulster plantation scheme was not isolated in the same way as Munster. Those settlers who came, whether ‘official’ or ‘unofficial’, all shared a common identity - deeply Protestant and loyal to the British Crown. At least if we are being relatively simplistic about it for the sake of coherence in a short post.

There were Protestants all over Ireland of course, but as a consequence of the Ulster Plantation only in the North East was there a substantial majority. Land confiscation and massive reorganisation of ownership following the defeat of a Catholic rebellion in 1641 created a system which came to be known as the Protestant Ascendancy in later centuries, ie. Catholic underclass (the majority of the population) acting as tenants to an elite Protestant landlord class in most of the country. Catholics where discriminated against in numerous other ways too, with the so-called Penal Laws imposing many restrictions that would only be lifted in the 19th century. There is a lot more to say on this, but for I am trying to be brief.

Only in Ulster did you have a really substantial number of Protestant tenants (of course not only in Ulster though), for the reasons highlighted previously. In centuries to come this would crystallise into a "settler colonial" identity, defined by staunch Protestantism, loyalty to Britain and an acute fear of Catholic revolt and rebellion. Almost a kind of paranoia. The 1641 Rebellion and the sectarian atrocities and killings which accompanied it served as a particularly influential "warning" in the minds of many Protestant settlers.

As the elite landlord class of the Protestant Ascendancy was finally dismantled through a combination of agrarian agitation and political reform in the 19th century, this left these areas of North East Ulster as the only part of Ireland still under any kind of real Protestant control. The simultaneous rise in a new form of Irish political nationalism in the latter half of the 19th and early 20th centuries saw the accentuation of the British identity of those in this part of Ulster.

Irish Nationalists began to seek home rule within the UK from about the 1860s onwards. This was a very limited kind of devolution actually, but it stoked Protestant fears of separation. The problem was of course that you had this substantial majority of Protestants in the North East who felt themselves to be distinctly British, and who were vehemently opposed to any form if independence (even as limited as Home Rule). So they tried to block Home Rule from happening for the whole country (for instance signing the Ulster Covenant in 1912 vowing to oppose it by any means, founding the Ulster Volunteer Force, and threatening civil war). This was called the Ulster Crisis and these Ulster Unionists were also backed by conservatives in Britain. Of course it wasn't confined solely to north east Ulster - the leader, Sir Edward Carson - was a Dubliner. But for obvious reasons, noted previously, Ulster provided the backbone to the Unionist movement. Only the First World War stopped things likely spiraling into civil war between supporters of home rule and Irish nationalists.

However, following the Easter Rising in 1916, the situation began to change. In the December 1918 general election, Sinn Féin won a massive victory running not for Home Rule, but an Irish Republic. They formed a breakaway government known as Dáil Éireann which declared total independence from Britain. As events escalated further away from Home Rule and constitutional politics with the outbreak of the Irish War of Independence (fought mostly in the south) unionists in Ulster began to realise that they couldn't stop independence for the whole island (since they were a minority), and so began to at least achieve partition for the area in which they did have a majority instead, ie. North East Ulster, the area which now comprises Northern Ireland.

So in 1921 the government of Ireland act created the modern state of Northern Ireland (six of the nine counties of Ulster). There are some other good posts providing an in-depth look at partition in greater detail, I can link them later (can't find it on ky phone). This post was simply intended to provide a very broad overview to someone unfamiliar. There is a lot lot more that could be said. Of course, if you have specific questions then fire away.