Through a Stockbroker firm or an on-line account. Obviously it's more expensive to use a firm of Stockbrokers, but if you know what you are doing try the on-line route.
Rory Gillen runs courses in the IFSC for investors if you want to educate yourself.
AFAIK, www.selftrade.co.uk are quite good and well priced. I don't use them at the moment as I prefer to use the broker route as I obtain a super service from the ones I use and have built up a very good relationship with the broker I use.
I'm not sure if Selftrade take on any new Irish clients , but the best way to find out would be to e-mail them.
I had an account in a stockbrokers in Ireland. Two accounts were opened, one in Sterling and one in Euro. I never purchased Sterling. I simply lodged the equivalent amount of my Sterling purchase to my Euro account. This was then offset against my Sterling trades.