I recommend you have a look around a few jewellers near where you live - get an idea of what you can get for your money. You'll find that some jewellers are way more helpful than others so when you come across a place that's helpful stick to them. They should know what your bedget is and treat you the same as someone with €500 or €50,000 to spend. Another good tip is to then arrange with them that when you return with your fiancee that they only show ye rings in your (pre-defined) price range. Good luck!
I got engaged last year, and got the ring in Diamonds and Gold Ireland (based in Galway, and now one open in Dublin). They specialise in engagement rings - and I found them to be much cheaper (for same quality - Look up the 4 C - Cut, Clarity, Colour and caret I think) than other jewelers that went to.
If you can make it to Newry Jack Murphy Jewellers were by far better value and quality than most jewellers in Dublin. We got a lot more for our money with them, best of luck...