June of 2015 plan is to fly into Dublin and rent a car to travel through Ireland for 4 days then ferry to Scotland for 4 days then down to London for 5 days. Turning the car over in London. At this point we just have lists of sites we want to see, no set time frame. I would like to know if having set accommodation is necessary or would we be able to find it when we decide we need it? We already have set accommodation in England.
I just returned from pretty much the same trip but in the reverse order. I find it's best to have reservations in the big cities. Ireland has the best answer for finding rooms with it's Irish Tourist Board. Every city/town of any size or importance has a ITB. If you need rooms, they will search and find them for you. Scotland and Wales have similar Boards, but they don't seem as ubiquitous. In Dublin, the Central Hotel is a great location, fun hotel, and, if you call them directly or email, you get a good rate. The place where we had a hard time finding rooms was in Wales, due to a Tom Jones concert. Who knew he could still bring in a sell out crowd? Have fun and drive carefully. Spend the extra for an automatic transmission car.
You'll have no problem finding a place to stay when you're ready to stop for the day.
You'll see B&B signs all along the way.
You've a flaw in your overall plan, however:
. Hertz is the only company that will let you take a rental car across the Irish Sea, and they demand about three hundred bucks per day for the honor (normally a car in either nation is less than thirty per day).
. The ferry isn't that great and there's no place to rent a car on the Scotland end (but there is in Holyhead).
. Figure on flying from Ireland to Britain. Ryan will do the job, read their rules and buy early.
I would be willing to "risk" not reserving B&Bs for the driving portion of the trip (for which you need two separate cars, as Ed described). That flexibility is easier when you're not too pressed for time, but the car also makes it work. When booking through the Tourist Office, it helps to be in town before 5 p.m. You can also book online or by phone from the road as you go, maybe a day or half-day ahead, using your guidebook and online resources.
If you don't care where you stay you will not have a problem but you may not be able to get the most popular or unique B&Bs with the best views, locations, hosts, breakfasts etc. Some are really outstanding and worth the effort in advance to secure. I always book in advance. Then again 95% of B&Bs in Scotland and Ireland are just fine. If where you stay is not paramount to you you can wing it and be assured of a decent place. Ed's warning about the car must be looked into however.