Actually, the opposite is true, for most of the time that Britain ruled Ireland. The initial involvement was started by invitation. Because Ireland was a patchwork of small kingdoms, Henry II was able to gain sovereignty very easily. So not really a struggle in the first place. England and Britain, apart from odd periods such as under James I and Elizabeth, pretty much left Ireland alone. It was only when Spain and France sought to use Ireland as a means of destabilising England/Britain that any interest was shown. Even Henry VIII did little apart from respond to revolts. There was a rebellion in 1640, but Britain's Civil War was developing so there was little response until the War was over, when Cromwell went to Ireland to avenge the massacres and restore order. Plague and famine helped a lot in that, and it wasn't much of a struggle. The Williamite/Jacobite War was essentially a civil war between two monarchs that was fought out in Ireland and Scotland, so wasn't really an invasion of Ireland as such. The French Invasion and the rising of 1798 that they sponsored was defeated as much by Irish troops as British. So, in conclusion, Britain/England didn't struggle to conquer Ireland, and only briefly actively opposed independence in the early 20th century. Ireland was only desirable in the sense that Britain couldn't afford such an important strategic territory to be in Spanish or French hands. Ireland was so weak militarily, politically and economically that if Britain hadn't taken it, another power would have.

I agree with Second_mate overall. Ireland was a very attractive territory for medieval England being relatively large, very fertile and having no unified government. However this was also a problem as England had to deal with the various Gaelic lordships separately. Fully conquering Wales was an incredibly expensive business because of the huge castle building programme it required and the crown was too involved in conflicts with France, Scotland and various civil wars to try that again. It was more profitable to retain control by proxy and suppress rebellions when they occurred.

The reformation is important as well. Ireland was the largest Catholic population in the British Isles and this made it potentially threatening to the Protestant kingdom which was constructed after the sixteenth century.

One problem in popular Irish history is that it tends to exaggerate British interest in Ireland. English / British governments that actually had a clear Irish policy (whether Cromwell or Gladstone) were quite rare.