Bewteen 1988 and 2007 we took nine bicycle trips (July-October) all over Ireland and the North, reserving our B&Bs for only our first and last nights. We chose our routes and overnight towns day by day and within a few minutes of arriving in town had secured a room by calling or knocking on a door or two.
The Internet has taken away all that flexibility and serendipity, as booking months in advance is now the norm, even for restaurants.
In 2022, I took the advice of a few locals I know, factored in the effects of Covid and retirements on B&B closures (c. 2007 and before there were some 2,500; today, perhaps a third that number) and the housing of some 50,000 Ukranian refugees in the countries' hotels and B&Bs and for a month-long solo bike trip made advance reservations for each night. It took a couple of hundred emails and phone calls and a full month to do so. Even then, only about a third of the B&Bs I contacted responded. In the end, though, I had a bed for every night.
Moreover, there are fewer restaurants today than in prior years. Many had closed permanently and others were short-staffed, which meant each was closed on a different day of the week.
The major downside to this, in addition to too much structure to the trip, was not being able to see and select my B&Bs before committing. Even so, 99% were excellent, save for one whose website presented a totally fraudulent picture (and was grossly overpriced and had no resident owners) and another where I was quite rudely treated. The owners of the latter, not surprisingly, are not Irish, whom I've found are the friendliest and most hospitable people in the world.
To decide whether or not to reserve in advance or to wing it then is a toss up. It may still be possible, but especially during the summer months the competition will be heavy.