In nine trips over the years, we've stayed in some 150 B&Bs all over Ireland and Northern Ireland, winging it every day except for our first and last nights in-country. Most of the trips were in July and August, a few in September and October.
On only two occasions did we have any difficulty at all securing a place for the night. In one case, we simply made a few more calls (well, about 10 or 15!) beyond the one or two usually needed for same-night stays for a Saturday night in a popular commercial town (Westport in County Mayo). In another, we just bicycled a few more miles than we had intended that day. In no case did we ever settle into a B&B later than 6 pm.
October is well into the low season, and you should find it quite easy to pick and choose where you want to stay. Researching any big concerts, etc. that might take place during your trip could help mitigate any doubts ahead of time. There are about 2,500 B&Bs on the island, making your daily choices even smoother.
I've never booked an Irish B&B via any middlemen, but imagine they would assess a surcharge for doing so. The in-country tourist boards do. Anyway, no need to book any more than 1-2 days ahead of time - or even the day of to stay flexible - unless you really want the security of knowing.
You can check out B&Bs on the Irish Tourist Board (ITB) website at: http://www.discoverireland.ie. ITB used to send out a free print version of its B&B guidebook and may still do so. I just couldn't find its New York City address.
If you'd like, send me the names and counties of a few towns in which you plan to stay, and I'll check my records for B&Bs we used. Keep in mind that we traveled exclusively in the coastal counties, not the island's interior.