Complex sentence: It was estimated in 2007 that 10% of Irish residents were foreign-born; most of the new arrivals were citizens of Poland and the Baltic states, many of whom found work in the retail and service sectors.
Split into simpler sentences: It was estimated in 2007 that 10% of Irish residents were foreign-born. Most of the new arrivals were citizens of Poland and the Baltic states, many of whom found work in the retail and service sectors.