Ireland's colonial history is brutal, long, and recent. It started with the arrival of the Normans in 1168 (there were others before then, but the Celts we think were fairly peaceful and merged into the population, and the Vikings never managed to take over the island -- they raided monasteries and the like, but eventually their settlements got absorbed as well.) The Normans (sent by Henry II from England) actually conquered and took over. The Irish fought back, and at times even managed to take back control, but for the most part, they were ruled bloodily by the English for roughly 900 years. One war ended up with a third of the native population dead or in exile. And of course, Ireland got hammered by bad winters and potato blight, which natural disasters were exacerbated by bad English absentee landlords, unfair tariffs, and later varieties of economic exploitation. In the second great potato famine, the population of Ireland dropped from 8 million to 4.4 million. HALF the population died or emigrated. It's frankly astonishing that Ireland managed to recover from that.
In 1922, the country was divided in half -- the mostly Protestant six counties in the north staying part of the U.K., the overwhelmingly Catholic south becoming the Irish Free State, and later, Ireland. Northern Ireland continued to have armed resisters (the IRA, which some would call freedom fighters and others would call terrorists) until very recently. Leading up to 1972 were a variety of resistance actions and brutal retaliation by the British government / local Unionist government, leading up to the events of Bloody Sunday. In that conflict, more than 3000 men, women, and children were killed, and as a result, from 1972 - 1999 (27+ years), Northern Ireland essentially had their government taken away -- they were under Direct Rule of a British Secretary of State.
Starting in 1999, a series of peace talks and power-sharing agreements led to the struggle quieting down. The population was also exhausted, I think -- sick of war, after so much blood and death. Colonialism destroyed the Irish economy, over and over again, and many Irish were tremendously poor as a result. Their native language (Gaelic), was also almost entirely wiped out and replaced by English. The IRA finally gave up armed struggle in 2005, and with the drop in violence and the tech industry resurgence, Ireland's economy has been doing much better in the last few years. Some of the kids are even starting to learn Gaelic again.
ADDED: Note from reader: "Gaelic" isn't used by native speakers to describe the language. We use "Irish." (Using "Gaelic" actually has colonialist, and offensive, overtones, and I know you didn't mean to do that.)"
I didn't know that, and it makes sense, when I stop to think about it. Am leaving it as written, just for clarity's sake, but will use Irish in the future!