Murphy, the most common Irish Surname.
The Surname 'Murphy' derives from the very popular first name 'Murchadh' which means Sea-Warrior. Some variations include: MacMurphy, MacMurrough, Morphy, Murphy, O'Morchoe,O'Murphy. The most common places to find these names would be in Dublin, Cork and Wexford.
Mac Murchadha meaning the son of Murchadh is almost totally exclusive to Ulster, where the family was part of the Cenél Eoghain, the tribe who claimed descent from Eoghan. Eoghan (Eugene or Owen) is a very common first name in Ireland and is also very popular in the McCarthy (MacCarthy, Mac Arthy) line)
Below is a picture of the Murphy Crest as well as the motto - "Fortis et Hospitalis" meaning "Brave and hospitable".
The Oxford English Dictionary makes a claim that Murphy is derived from the first name Morpheus, this is possible but not much evidence exists to support this theory.
The modern day variation, Murphy is the most common Irish surname today.