Genetic research suggests that the earliest settlers migrated from Iberia following the most recent ice age. After the Mesolithic, Neolithic and Bronze Age, migrants introduced a Celtic language and culture. Migrants from the two latter eras still represent the genetic heritage of most Irish people. Gaelic tradition expanded and became the dominant form over time. Irish people are a combination of Gaelic, Norse, Anglo-Norman, French, and British ancestry. The population of Ireland stood at 4,588,252 in 2011, an increase of 8.2% since 2006. As of  2011, Ireland had the highest birth rate in the European Union . In 2014, 36.3% of births were to unmarried women. Annual population growth rates exceeded 2% during the 2002-2006 intercensal period, which was attributed to high rates of natural increase and immigration. This rate declined somewhat during the subsequent 2006-2011 intercensal period, with an average annual percentage change of 1.6%. At the time of the 2011 census, the number of non-Irish nationals was recorded at 544,357, comprising 12% of the total population. This is nearly 2.5 times the number of non-Irish nationals recorded in the 2002 census , when the question of nationality was asked for the first time. The five largest non-national cohorts were Polish , UK , Lithuanian , Latvian  and Nigerian  respectively.
Question: Who accounted for the least of the non-national population observed?
Answer: Nigerian