http://www.komplett.ie <-- Good for everything really, located in the Netherlands shipping takes around 3-5 days and postage is some what cheap... around 12-35 €. Good deals on packages such a a E6600 + Motherboad + Ram..
http://www.overclockers.co.uk <-- This place tends to have more high - end products, but they get new hardware in very fast, they where one of the first few online retailers to get the Core 2 duo CPU's in.. Located in the UK, postage is around 15£ and takes 2-3 days.
http://www.ebuyer.co.uk <-- Over the years this site has changed its stock alot, they still are good but tend to be pricey... but there is often good bargains to be found! Located in the UK, postage varies to Ireland according to weight and can sometimes take up to 2 weeks.
http://www.dabs.co.uk <-- Great for RAM and computer cases, also located in the UK, they have a fixed Irish shipping rate.. Personally i haven't Tried anything other then Ram.
http://www.shop4memory.com <-- is great for memory and is based in Celbridge
http://www.pixmania.ie <-- is good too, nice alternative to Komplett who can be hard to deal with. note: they may ask for a scaned bill of your address to prove you are who you say you are, so have one ready just in case!
http://www.linitx.com/ <-- UK based, specializes in VIA ITX systems but also sells all other manner of hardware that is not always easy to obtain, useful for HTPC builders etc
http://www.hitide.ie <-- Irish bassed, some good usefull stuff on this site, EG ram heatsinks etc, very fast shipping though it can be expensive.
If you have any more sites, please add them!
http://www.marxcomputers.ie/ <-- They have reasonable enough prices for most things, excellent customer service and based in Dublin (unlike the majority of the others, so cheaper or no shipping if you can pick it up). I have dealt with them on numerous occasions. Class act all the way.
http://www.alphacool.de/xt/index.php/language/en/ <--Excellent to deal with for watercooling and modding stuff
http://www.lsdiodes.com/<--If you need LED's then LSDiodes can't be beaten. Quick delivery even though they are in US. Freebies if you order in bulk too!
http://www.elara.ie <-- Bassed in Ireland, very good prices and has the option to Pickup to save on delivery.