There are several different types of deeds that might have been registered (John Grenham describes them clearly here.) The most useful documents genealogically are probably marriage settlements - hence the title of this post. Essentially, these were the pre-nups of their day, making provisions for the future couple and others. Within some of these, I got some major breakthroughs with my Jones and Smith ancestors (I kid you not!) as family relationships were detailed in relation to lands.
Often reference is made to earlier deeds so it can become quite addictive hunting down the paper trail and because you can look at the transcripts yourself, you are not having to wait or pay for documents to be fetched for you only to find out they don't apply to your family. There is no guarantee that you will find anything for your family - the Registry dates from 1708 but it is generally limited in scope to a minority of the Protestant land-holding families (Again, see John Grenham's excellent "Tracing Your Irish Ancestors" for a better explanantion.)
Some deeds are short enough to transcribe (you may not take photographs) but it is worth ordering longer ones albeit at 20 Euro each. What you get is an actual A3 copy of the "Memorial" which will probably have your ancestors actual signatures!