I do not believe that you can be fired for getting a tattoo, but they could make things "difficult" for you at work so that you then decide to leave. As other people have said the tattoo may then limit your ability to get another job.
Although tatoos are more accepted these days, some employers still frown on them being visible to the public.
You also said that you have waited a year, have you really thought of what you will think when you turn 40 or even 50 and see a Harry Potter theme tattoo on your hand, I honestly think it is then that you may regret having it put some where so visible