Claim: Ireland has relatively low-lying mountains.

Ireland -LRB- -LSB- ˈaɪərlənd -RSB- Éire -LSB- ˈeːɾʲə -RSB- ; Ulster-Scots : Airlann -LSB- - scoˈɑːrlən -RSB- -RRB- is an island in the North Atlantic . It is separated from Great Britain to its east by the North Channel , the Irish Sea , and St George 's Channel . Ireland is the second-largest island of the British Isles , the third-largest in Europe , and the twentieth-largest on Earth .   Politically , Ireland is divided between the Republic of Ireland -LRB- officially named Ireland -RRB- , which covers five-sixths of the island , and Northern Ireland , which is part of the United Kingdom , in the northeast of the island . In 2011 , the population of Ireland was about 6.4 million , ranking it the second-most populous island in Europe after Great Britain . Just under 4.6 million live in the Republic of Ireland and just over 1.8 million live in Northern Ireland .   The island 's geography comprises relatively low-lying mountains surrounding a central plain , with several navigable rivers extending inland . The island has lush vegetation , a product of its mild but changeable climate which avoids extremes in temperature . Thick woodlands covered the island until the Middle Ages . As of 2013 , the amount of land that is wooded in Ireland is about 11 % of the total , compared with a European average of 35 % . There are twenty-six extant mammal species native to Ireland . The Irish climate is very moderate and classified as oceanic . As a result , winters are milder than expected for such a northerly area . However , summers are cooler than those in Continental Europe . Rainfall and cloud cover are abundant .   The earliest evidence of human presence in Ireland is dated at 10,500 BC . Gaelic Ireland had emerged by the 1st century CE . The island was Christianised from the 5th century onward . Following the Norman invasion in the 12th century , England claimed sovereignty over Ireland . However , English rule did not extend over the whole island until the 16th -- 17th century Tudor conquest , which led to colonisation by settlers from Britain . In the 1690s , a system of Protestant English rule was designed to materially disadvantage the Catholic majority and Protestant dissenters , and was extended during the 18th century . With the Acts of Union in 1801 , Ireland became a part of the United Kingdom . A war of independence in the early 20th century was followed by the partition of the island , creating the Irish Free State , which became increasingly sovereign over the following decades , and Northern Ireland , which remained a part of the United Kingdom . Northern Ireland saw much civil unrest from the late 1960s until the 1990s . This subsided following a political agreement in 1998 . In 1973 the Republic of Ireland joined the European Economic Community while the United Kingdom , and Northern Ireland , as part of it , did the same .   Irish culture has had a significant influence on other cultures , especially in the fields of literature . Alongside mainstream Western culture , a strong indigenous culture exists , as expressed through Gaelic games , Irish music , and the Irish language . The culture of the island also shares many features with that of Great Britain , including the English language , and sports such as association football , rugby , horse racing , and golf .