With Eircom, I'd probably recommend connecting through the cable directly if possible. It's a while since I had Eircom, but the drop in BB from contention rates was annoying. In fairness you'd still get that with wifi and I never really had any signal drops (very occasional) with Eircom wifi routers.
However, there really is little difference with wireless and direct in terms of speed and connectivity. Depending on where the wireless router is you should get a good signal in the house (position of the box against solid walls can cause a few blackspots).
However, if I recall correctly, to connect wireless to the XBOX you need a separate adaptor, wifi isn't standard. Might be different now, but doubt it.
Set up is a doddle as long as you have the password for the wifi and would be done in minutes.