There’s a long and interesting article about common Irish surnames and their derivations on the Irish Medieval History page on facebook. It’s got some snippets of information I wasn’t aware of either. I’d always assumed anyone Irish with a French-sounding name had Anglo-Norman ancestors, but apparently not:
Assimilation is the name given to the process of substitution with foreign names of similar sound or meaning like these French examples. Ó Lapáin became De Lapp, Ó Maoláin became De Moleyns, Ó Duibhdhíorma became D’Ermott. Molloy (O’ Maol an Mhuaidh) and Mulligan (O’Maoláin) became Molyneux.
There’s also a link to an interactive map of Ireland showing surnames by county from 1890 census. Label size represents relative birth counts. It can be accessed directly here: http://storymaps.esri.com/stories/ireland/
Of definite use to genealogists as well as history buffs, I think.