There are some reasons why it may not be a good idea to arrive without booking in advance.
Main reasons are Availability and price.
Availability:
Especially during the peak tourist seasons or when there is a popular event going on, accommodation in some locations (Kerry Galway, Dublin and others) can become fully booked up quite fast and it can be very difficult to find suitable accommodation at short notice.
Price:
Usually the later you book the higher the price is. Which is why I always encourage people to book well in advance. Nowadays there are lots of options for booking without deposit with the possibility of cancelling right up to the last day.
See here for more information on this and other tips on how to get cheaper accommodation.
https://dublinovernight.com/how-to-pay-less-for-your-dublin-hotel-accommodation/