Ring around. You can get good discounts if you have car or health insurance with someone that also offers home insurance. Make sure you have your rebuild costs right (the chartered surveyors website have rebuild costs). Consider what excess you want to pay. If you have an alarm and go for the alarm discount, you have to have the alarm on if the house is broken into to be able to claim (unless you are just popping to the shops - I asked about this and didn't get a more specific answer than that). Smoke detectors will also get you a bit of a discount.
I ended up with AA and got about €100 off what I was paying the year before, but I also went with them for car insurance. Best place to start is either Quinn or Hibernian if you have health insurance with them or whoever does your car insurance. But do ring around. Every year (for car insurance at any rate), I call AXA, Alliance, AA, Quinn, Hibernian, FBD and have a notebook with phone numbers and quotes and a list of cover available so you remember who told you what! When I get the lowest quote, I call back the people I'd be most inclined to go with and tell them of my lowest quote - I've always gotten either a lower price again or better cover (e.g. no claims protection for free). It takes a full morning, but there's nothing like saving money on insurance!
While 123.ie will give you some general info and I'd check them once I'd gotten quotes from the primary source, I find that the actual providers have more flexibility.
Sprite
p.s. Also, if you are ringing around, make sure to tell them that you don't want to be contacted afterwards or else they will inundate you with junk mail.