As an example I spent ages last night searching for names in their Irish deaths 1864 to (some date I can't remember) and got 0 results for every name I searched. Frustrated, I searched for 'Mary' no date, no other boxes filled in - and, still no results. How can that be? No deaths for anyone called Mary within a huge time frame in the whole of Ireland. Maybe I was doing something wrong?

FindMyPast do not say how many of each record they have (as Ancestry and Familysearch do). This gives an idea of how extensive the particular database is.
I presume many of these record collections are extremely small and are used as 'padding' to make it seem like they have a huge number and variety of records, when in actual fact most of them are of little or no use to anyone.