Just read a few chapters of John Relethford's Reflections of Our Past: How Human History is Revealed in Our Genes, very interesting book (broader sweep than most works in this genre)-but I found the chapter titled Invasions, Settlements and Irish History, somewhat surprising. The author recounts his analysis of both anthropometric & genetic data, and asserts that:

1) there is a west to east cline or gradient.
2) four midland counties, Roscommon, Leitrim, Longford and Westmeath are outliers from the main axis.

My first thought was that these counties were genetically isolated from the impact of various exogenous settlers, whether that be pre-historical (Fir Bolg? Milesian?) or more recentl (Viking, Norman, English, etc.). But, Rethelfod says that the anthropometric & genetic data points to a Scandinavian connection! Look at the map of Ireland and you note that the Shannon river allows an avenue of intercourse from the coast up into the heart of Ireland, and there are records of Viking warlords making their way inland and settling down.