The recent discussion of Irish surnames listed many sources of variation, while somewhat soft-pedaling all the complications introduced by anglicization. In my case the surname "Fallin" (dating back to a first immigrant to Virginia before 1671) was a mystery. YDNA analysis solved that mystery by showing that, while it resembled (and was pronounced like) "Fallon," all my primary DNA connections were with people with "Phelan," "Whelan," and similar anglicized versions of Ó Faoláin. That result, while it didn't provide an answer to why we left Ireland for Virginia (possible with a stopover in the West Indies), it did provide me with a positive link to a new Whalan "cousin" in South Australia with our closest common ancestor a mere 450 years back! It may not work for everyone, but when it does, it's clear as day.
Walnut Creek, CA