While not doing the same routes as you I did do the 'Grand Tour' of Ireland and 'Lon Las Cymru' a few years ago. Did it from North to south, though. Then rode out to St. David's through Swansea, Llanelli and Pembroke. North along the coast to and through Fishguard, Aberystwyth, Harlech, Caenarfon, Bangor, across the Menai Bridge and across Angelsey to Holyhead. There's a B road that goes across Angelsey which is much more rural than the A5...can't remember the rt #, though.

The first half was Dublin to Dublin counter-clockwise. South through Wicklow, Arklow, Wexford, Dungarvin, Waterford, etc. to Killarney. Included was the Ring of Kerry. Missed Galway as I elected to go to the Aran Islands, instead and took a ferry to Rossaveel. Then north through Leenanne and up to Westport, Derry, Balleycastle, Larne, Belfast, Slane and on to Dublin. Then took the Stennaline ferry across to Holyhead.

Was in each country for a month. My axiety level was the same as yours. Was I in good enough shape? Did I pick the right bike? Was my passport going to arrive in time? On and on. Then a guy whom I came to know through our lbs told me not to worry, "They've got indoor plumbing over there and if you're not fit for the tour in 2 weeks you will be." That relaxed me and made me laugh at myself. I'd never been out of the States before and here I was going on a 2 month bike tour. I was supposed to be a little nervous. You'll be fine. Have fun.