is correct as regards changing one's name.  As per the reference he supplied, in the Republic of Ireland, all that is necessary is that one begins using the new name, and that it not be done for fraudulent purposes.  Doing so is extremely common - how many thousands of women, for example, take their husband's surname on marriage without getting their name changed by deed poll?  Changing one's name in this way is an entirely different thing from having one's Registration of Birth amended, where specific statutes apply.  From the GRO's website: "Changes to the Register of Births are almost impossible to effect and generally are only done if it can be proved that a clerical or factual error was made at the time of registration."  So one can change one's name in common usage from, e.g., George to Jack, but George would remain on the Register of Births, unless you also changed it by deed poll.  My understanding on the case of the transsexual who wanted to have her birth registration amended was that she wanted the gender changed (though I'm open to correction on this).  As an aside - I queried Eamon O Cuiv's use of the letter 'V' in his surname on his article's talk page, and someone pointed out he conveniently drops the 'O' from 'O Cuiv' for listing on ballot papers...