The long-term causes have played an important part to the conflict in the current state of Ireland.
Ireland was once a prospering country many centuries ago when the Gaels were amongst the first settlers. The Gaels had imposed their language, customs and people to Ireland and made it the original way of life. Up to today these have had an everlasting effect on Ireland up to today and is regarded as important part of the Irish heritage to some Irish. The Nationalists refer to this time as the "golden era" when Ireland grew without the foreign political interference and these are the ideals to which nationalists strive to restore. That was a time when Ireland was 1 country, 1 people. The next people to join Ireland were the Norman's, they made little difference to Ireland as they themselves adapted to the customs of the Gaels. At this point the only foreign interference to Ireland was the British control over Dublin, known as the Pale.
By the British this was the only place believed to be civil and clean, the Pale was surrounded by a wall. Up to this point Irelands people are defining themselves and settling in, Ireland is beginning to culminate into what it is now. The Gaels are of most importance as they are referred to the "true Irish". At this point Ireland was 1 people split into different kingdoms, a peaceful place. This is why nationalists call this the golden era.
Britain was a growing empire and had to make sure the surrounding regions were secure. Instead of fighting the Irish they used a different means to gain control, plantations. They planted loyal Protestants in Ulster mainly, and gave them good land and housing previously occupied by Catholics to settle which is still a problem today.