(Based on your follow up questions, I am going to assume you mean was this the UK's first colonial effort in Ireland, not first ever colonial effort.)

First of all, the term "UK" is a bit anachronistic, as the term was not officially used until 1801 when Ireland was officially brought into the Union, and the earliest form of a "united kingdom" did not exist until the 1707 Act of Union that bound together Whales, Scotland, and England.

To directly answer your question, Cromwell's effort was by no means the first English attempt to colonize Ireland. The Normans made an effort in the 1100s, and Henry II made the conquest somewhat official by signing the 1175 Treaty of Windsor with the high king of Ireland, Rory O'Connor. It is important to note that while the English saw this as binding , many native Irish people did not really recognize the high king as having the power to do this. At any rate, Norman/English feudal rules gradually became the law in limited pockets of land over the next couple hundred years. However, starting in the mid 14th century, English rule began to rapidly decline, and a surprising and unexpected rebirth of the Gaelic culture and language occurred. However, a large percentage of the Irish population remained Catholic during this time period. English rule was basically limited to Dublin and its surrounding areas until the mid 1500s.

In 1542, King Henry VIII was declared "King of Ireland" and launched a series of campaigns to conquer the whole of Ireland and strengthen/confirm English rule of the island. (We should note here that this is the same King Henry who had created the Protestant Church of England and broke away from the Catholic Church, a fact that will have dire consequences for the by now devoutly catholic Irish populace. ) Over the next century England embarked on what we now call the "Tudor Conquest of Ireland" which included multiple military campaigns that eventually conquered the entire Ireland. There was also an effort to settle the land called the "Plantation of Ulster". Ulster is a region in the North of Ireland that would become a hot bed for violence as recently as the 1990s, as conflict between Irish Catholics and English protectants frequently spiraled out of control. In fact, it is three counties of Ulster that make up the UK's part of Ireland today. These seeds were planted in the Plantation of Ulster plan, which stole land away from Gaelic owners and put it in the hands of wealthy English ,protestant- and often absentee- land owners. A direct result of of this land re-allocation was the creation of a protestant towns in Ulster, and a wealthy ruling class of protestant land owners.

In 1641, Ireland rebelled and for a brief period they had were ruled by the Irish Catholic Confederation. It was this rebellion that prompted Cromwell's brutal campaign in Ireland starting in in 1649 and lasting until 1653.

In response to your second question, as we have noted above protestants' had already displaced thousands of Catholics in specific regions of Ireland, namely Ulster. Cromwell did not really bring protestant settlers, he only brutally put down a rebellion and reconquered Ireland, paving the way for the continuation of a small number of English protectants to control large tracts of land and be the majority in certain areas of Ireland, like Ulster. Exact numbers of "displaced" people is not nearly as important as the fact that it is estimated that there was anywhere between 15 to 83 percent drop in the Irish population as a result of the conquest and the ensuing famine.

The impact of Cromwell's conquest was massive as it was the proverbial straw that broke Irelands back. The conquest made sure that the English remained in control of Ireland for the next three and half centuries. Cromwell also helped reinforce the sharp divide between the Irish Catholic and the English protestants on two fronts. The first was, obviously, religion. Religious tensions would ignite violence time and time again over the ensuing centuries, with the protestant minority often having to rely on British military support to regain control. The second front was that of land ownership. The English backed protestant landowners who owned large plantations and often callously exploited Catholic Irish workers and small land owners were left in place and even reinforced by Cromwell. The divide between big and small land owners would get worse and worse as the centuries dragged on. It is no surprise that some of the earliest Irish demands of England were land acts that would reallocate land back to native Irishman. Time and time again, England would act far to late, and eventually, after multiple violent and terror filled rebellions, most of Ireland would break away in the 1920s. Of course, the troubles would kick off in the late 1960s, and when the the dust settled the mostly Protestant northern most part of Ireland would become Northern Ireland and is to this day a part of the UK, while the rest of the island is the Catholic majority Republic of Ireland.

I hope this helped answer your question. An excellent resourse to learn more is "Ireland: A Concise History from the Twelfth Century to the Present Day" by Paul Johnson, which focuses mainly on the relationship between Ireland and England starting in the 12th century.