Ammo: you don't have to have a full-face hat in Ireland. As long as it's proper spec,the only thing we don't like is polycarbonate, but only a nutter would use such a hat. Your ACU stamped helmet wiil be OK, subject to the above.
I use an Arai SZ with tilting visor - no problem.
As to the Ulster,if you ride a Classic (apologies if you don't!!) the Classics run under a National Permit.
You'd enjoy Dundrod . . . . . .
Hope this helps.