The English were still being very very nasty up until the mid-19th century, and the 17th and 18th centuries had been brutal http://www.globalresearch.ca/the-irish-slave-trade-the-forgotten-white-slaves—so 800 years would not be necessary. The older generation would remembering what their grandparents had experienced and relayed from the few previous generations would have been quite ample.
That it has been so swept under the rug that there is not a realization of how relatively close it is would make those few who do remember quite bitter.