We ate 15 Irish breakfasts in Ireland, and even with all the subtle variations from one B&B to the next, never once did mushrooms appear on the plate, so I'm not sure what you're going to use them for. As for the tomatoes, the best thing to do is to keep it simple. Most places used a smallish tomato, about an inch and a half in diameter, sliced in half and then lightly griddled on the cut side. Some left it on the griddle long enough to brown the edges a bit, others just warmed the tomato ever so slightly.
I admire the lengths to which you are going for authenticity, but if cost is ever an issue, Canadian bacon, although not exactly
like Irish bacon, is a suitable substitute and more readily available. The black and white puddings we were served were cut into medallions about a half inch thick and griddled. A standard portion seemed to be two medallions of each variety. While I did eat the stuff, and even enjoyed it, I think any more of it would have been too much.
We're in full agreement on the soda bread. My Irish colleen of a wife makes a fine version which we enjoy every St. Pat's day and for how ever long it lasts beyond that. Soda bread is quite tasty as a base for corned beef sandwiches too. I actually prefer a soda bread made with currants for that purpose. The sweetness of the currants is a perfect compliment to the salty corned beef and tangy mustard.
Of course if you want real
authenticity in your Irish breakfast, skip the soda bread and try to find an Irish brown bread that's been baked in a peat fire. OMG, that is the food of the gods brother! The B&B host would serve a basket of toasted white bread and slices of brown bread. By day three of the trip we were telling them to just bring the brown bread cause the white toast would go to waste.
Other variations on breakfast included, baked beans and potato pancakes. Also available were several dry cereals, and the best meusli I have ever tasted, nothing like what we get over here. There was also fresh fruit, juice, numerous varieties of yogurt, coffee and tea.
Good luck with you're St. Pat's day festivities. Sounds like you've got all the ingredients you'll need.
P.S. A quick tip on cooking the Irish bacon: It is so lean you will need to add some oil to the pan before throwing the bacon in, otherwise it will stick to the pan. I speak from painful and expensive experience.