Colonialism has had many negative effects on Ireland since the Anglo-Norman conquest in the twelfth century when English barons grabbed lands in an area including Dublin and its surroundings known as the "Pale". The Tudors in the sixteenth century expanded and intensified English control over Ireland, disarmed local Irish aristocrats, and tightened control over the entirety of the island.
These elements of the Anglo-Norman and English conquest of Ireland had many of the typical elements of colonialism. The Irish were a Gaelic speaking people and the English spoke English. From the time of Henry VIII on, the English were Protestants and the Irish Roman Catholics. This led to driving both Roman Catholicism and the Celtic language and cultural traditions underground.
Internationally, many Irish groups opposed to English rule allied themselves with the enemies of England (such as Spain and France). These tensions led to increasing oppression of Irish Catholics, including confiscation of land and property, criminalizing Roman Catholic worship, making English the language of government and commerce thereby disadvantaging Gaelic speakers, and restricting access to education to Protestants. The 1801 Act of Union between Britain and Ireland, while giving the Irish some representation in Parliament also attempted to suppress the nascent Irish nationalist movement.
The main effects of colonialism were impoverishing the Irish, suppressing Celtic cultural and linguistic traditions, centralizing power and wealth in the hands of the Protestant minority, and creating civic unrest which continues to the present day in Northern Ireland. The combination of poverty and oppression also lead to massive emigration, at certain times amounting to 40 percent of the adult population, and a large diaspora that still maintains ties to Ireland and influences Irish politics and culture.
One particularly significant piece of evidence of this cultural hegemony is that most of the Irish today are functionally monolingual speakers of English, with only a small minority knowing the Irish language.