Where can I buy a custom pc in Dublin?

http://www.komplett.ie are probably your best bet. They do a number of bespoke systems (within a limited choice of components) for some very reasonable prices. Oddly, they don't appear to have any desktops at the moment - possibly something wrong with their website

Failing that, you could give constructing your own PC a shot. You don't need any knowledge (just a little research) and there are loads of people willing to help and guides online. It's just like lego for big boys.