The really short answer: English invasions began in the 12th century and set off more than seven centuries of Anglo-Irish struggle marked by fierce rebellions and harsh repressions. A failed 1916 Easter Monday Rebellion touched off several years of guerrilla warfare that in 1921 resulted in independence from the UK for 26 southern counties; six northern (Ulster) counties remained part of the United Kingdom. In 1948 Ireland withdrew from the British Commonwealth; it joined the European Community in 1973. Irish governments have sought the peaceful unification of Ireland and have cooperated with Britain against terrorist groups. A peace settlement for Northern Ireland, known as the Good Friday Agreement and approved in 1998, is being implemented with some difficulties. For more info: http://en.wikipedia.org/wiki/Ireland

And basically it's the only part that Britain took over

The even shorter answer. The British Isles were all british ruled until the rebellion mentioned in the slightly longer shorter answer. Ireland gained independance from British Rule but the northern 6 counties comprising northern ireland (not ulster) refused to join the republic

because it is a seperate country with a different govt and currency. and long may it last

up until 1921 England scotland wales and Ireland formed the United kingdom of great britain and Ireland then in 1921 the southern 26 counties in Ireland declared Independence while the 6 Countie of Northern Ireland voted to stay part of the United Kingdom people in Northern Ireland today are part of the U.K because we want to be part of the United Kingdom it amazes me how much especially americans are ignorant and think we are being "occupied" which we are not of course the only reason why we are part of the United Kingdom is because we the people of Northern Ireland wish to be so there is a Minority of people in Northern Ireland whom want to Unite with the Republic of Ireland which is a seperate Country from NI and indeed the rest of the United Kingdom we like Our counterparts in the Rest of the United Kingdom are loyal British Citizens people born here can have both a British Passport and a Irish passport or both ..unity will only happen if and when a majority votes for it may i suggest that rather that listening to hear say that those of you go and read the Good friday agreement which states that "it is hereby declared that Northern Ireland in it entirerity remains part of the United Kingdom for so as long as the majority of the people of Northern Ireland wish it to do so " and basically if we in the future vote for a United ireland then that will happen but at the Moment no one in Northern Ireland or in the Republic of Ireland wants a United ireland

The northern ireland was granded home rules(indipendance)from mother country in 1920 and tottall separation from ireland and olso i think becouse it was the olnly place of the ireland that remaind loyal to the crown

If the Nothern part of Ireland chose not to join the Southern Republic , why the ding dong?

because the irish sea is in the way, climate change!