Don't know about Ancestry... but FS is very difficult to explain, there are a few posts on the Ireland Resources Section explaining how to use them which will most probably be under Reg of Wills/Deeds threads.
I'm used to them having been to Reg of Wills/Deeds in Dublin for too many years and having them explained. They are not in numerical sequence many times and page numbers don't match the page numbers in reference.
For instance, I was looking for irwin/ormsby stuff so obviously they would be under 'I'
ehhh.. No.
When I looked at the 'I's there were only a few Surnames....then I realized many were under J
So one needs the check E for Erwin, Ervine I for Irwin, Irvine and if none under I then check J for Jrwin, Jrvine because of the way some wrote their I's and others wrote the J's so these are often lumped together.
For Stewart... check Stewart, Steward, Steuart, Steuard, Stuard, Stuart etc etc...
The Index is here...
https://www.familysearch.org/search/catalog/185720?availability=Family%20History%20Librarythe early ones are for many years e.g. Grantor index O 1708-1785 but as one goes through the following years you can see how difficult or virtually impossible to find stuff without having a reference to what year something happened!
So you find one you want to look at...the first column matches the volume (v) you need so you scroll down catalog until you see Deeds, etc., v. e.g Deeds, etc., v. 158-159 1752-1753
if you need 159 you need to scroll to middle of film then look for the black index card at start of film, double check it says 159 then look after black card for the reference number you want. At that stage you can hope it is in the sequence lot....if not all one can do is check page after page after page until it is found.
Simple enough when you get used to it..... so end of tips!
attaching a snippet from Index so you can see without a year extremely long process!