Ireland was a part of the UK in the saem way that Hawaii is a part of the US - Ireland had full democratic representation in the UK parliament from the Act of Union in the early 19th century.
Pitt caused deep alienation and suspicion amongst the Catholic community by reneging on part of that deal by constraining Catholics' land rights and democratic participation - this was clearly morally wrong and politically damaging, but needs to be understood in a context where it was less than a century since the last counter-reformatory invasion of Britain.
Much was made of the cruelty of the English governing class during the Potato Famine, but the reality is that the rich ruling class didn't care that much about those starving in England, Scotland or Wales either. It was - IMHO - more a case of the ruling class being incapable of empathy with the poor than racial discrimination. None of that diminishes the very real suffering of the Irish people at that time.
Liberal governments in the UK were instrumental in pushing through a series of reforms increasing the rights of Catholics in Ireland (and the rest of the UK where they were also constrained, albeit to a lesser extent) through the 19th century and there were three attempts to provide Ireland with full autonomy and Home Rule, each time defeated by the Conservatives who relied on the Irish vote to increase their chances of power in the UK parliament.
The last came in the 1906-1910 period, and there has been intense debate as to whether successful passage of that Bill into law would have prevented the easter rising and Irish independence. Certainly undivided Ireland's position would have been similar to that of Australia or Canada; the principle doubts surround the reaction of the Protestants in N Ireland to the inevitable restrictions on their discriminatory treatment of Catholics in the six counties.
The rest, sadly, is history.....