The most important thing is the topic. You have to find something that really grabs you. Not a general area of philosophy, not a particular philosopher, but a real problem, based on a deep disagreement in the literature. Think about some of the MA essays that really grabbed you, on topics where you have a lot more to say. Read around the topic to look for gaps in existing literature. Contact a member of the UCD Philosophy faculty to talk through the problem with them.
Once you find the right topic, writing a doctoral dissertation can be an exhilarating experience, and can transform your understanding of the world, and of you place in the world. But your decision to attempt one is a very personal one, and requires caution and self-reflection. Three big things to bear in mind.