As far as I know no operator provides 3G/HSDPA SIM only packages in Ireland (at least not to retail customers) so you might be stuck? I have a 3G/HSDPA slot in my latest laptop but could not find a way to get a (pay as you go) SIM only package for it.
Sorry, I didn't state this - I already have 3's 3G broadband with a modem and would continue to use it e.g. in the UK or when offsite at work or at parents etc. I assumed I could just swap it in and out between the modem and router as required?