Don’t spoil your vote

• Check that you haven’t skipped a preference or written two of the same preference. (The duplicate or missing preference and everything after it would not count. If you want to give two candidates an equal preference, you can’t.)
• Each ballot paper is separate, and requires its own first preference, optionally a second preference, and so on. (Don’t write a second preference on one ballot paper that is supposed to follow a first on another.)
• Write clearly. If your handwriting is difficult to read, a vote is important enough to write slowly and carefully. (In every election, there are ballots where agents can debate what digit a certain mark is. For example a “1” with a serif sometimes looks like a “7”.)
• Don’t mark an “X” or line, etc. beside the candidates you are not voting for. (Sometimes a line looks like a “1”, or it can be argued that the “X” was intended to be a vote for that candidate, especially if it is not against all candidates that don’t have a valid preference).
• If you make a mistake in filling in the ballot paper, you can return it to the officials at the table where you got it, and get a new one. (You could cross out everything on it to conceal who you were voting for and ensure that the ballot would not be counted if it ended up in the count in error.) Writing a correction on top of a wrong digit may spoil the vote by making it difficult to read. (Sometimes when a digit is written over another, it is not clear which is the corrected one.)
• Don’t write anything unnecessary. That includes slogans, political statements, etc. The law is that any superfluous mark spoils the ballot. (A lenient interpretation may be taken, but any word written on a ballot paper (or picture drawn on it!) usually spoils it). Such writing is seen by a few count staff, a few tally people, a few more agents at the adjudication of doubtful ballots, and if they are particularly funny, tallypeople will tell others about them. A comment on a discussion forum (e.g. Boards.ie) would be seen by much more people and is a much more effective political statement. (It might influence someone’s vote.)

You should write nothing but digits.
(A tick or an “X”, while technically wrong, is usually accepted, and treated as a first preference, provided that there is only one of them, and no other markings).

Also, check that your ballot has the official mark stamped on it (the presiding officer stamps it just before giving it to you). Each pin of the stamp must at least make an impression on the paper.