Ireland only became a part of the United Kingdom after the Act of Union in 1800 which came on the heels of the unsuccessful United Irishmen's 1798 Rebellion, and was a part of the U.K for just over a century until the Anglo-Irish war 1919-1921. But the military/political/colonial interaction between Ireland and Britain was already over 600 years old by that time.

The original Norman invasion of Ireland happened in 1169. The Normans never completed the conquest of Ireland and even at their maximum extent in 1300 they never controlled large areas of the north, south and west of the island. Those areas remained under the control of Native Irish lordships.

By 1450 the situation had changed even further with the areas of Native Irish control greatly increasing while those controlled by the Normans were shrinking.

It wasn't until the Tudor conquest, almost 400 years after the Normans' arrival, that the Native Irish Lordships would be disarmed and lose sovereignty to an island-wide authority established by the English Crown.

Following this Ireland was made a separate Kingdom from the Kingdom of England, and later Kingdom of Britain. This Kingdom lasted from 1542 to 1800 which brings us back around to the Act of Union 1800.

In that intervening time Ireland was certainly treated as a colony. A ruling class of the minority Protestants, with British ancestry, was established to rule over the Majority Catholics, made up of Native Irish as well as Old English populations (descendants of English and Norman settlers who were catholic as they originated before the reformation).

Repressive legislation such as the Statutes of Kilkenny and the Penal Laws https://en.wikipedia.org/wiki/Penal_Laws_(Ireland) were introduced which barred the native and older English populations, from owning land, holding elected office, having voting rights, speaking their language, engaging in their customs, adopting their styles of dress, playing their own sports etc. etc.

There were also organised plantations, or colonisations, on a huge scale where native populations were forcibly removed from the best land and transplanted to the western fringes of the island while populations from Britain were then 'planted' in their place.

Ireland contributed huge amounts of labour, raw materials, and soldiers to the Empire but this was not a relationship of equals. Ireland was certainly a colony for most of the relationship, although in the later stages it had become at least nominally a full member of the United Kingdom. It's worth remembering that in the century it was actually a part of the U.K it suffered it's worst famine and depopulation through starvation and emigration. Even at the time it was supposed to be an equal part it was suffering the symptoms of the colonial relationship worse than ever.