Here is the method I used successfully.

STEP ONE: if you know the townland, order LDS film no. 865092. This lists all of the townlands and their corresponding county, barony, civil parish, county district and electoral division. At the end of the listing for each townland is an entry "Number in Table VII of Census County Book, 1901". This number will be very useful, as it is the specific district number in the census where your townland will be found.

STEP TWO: order LDS film 990232. (You can do this at the same time as you order the film above, as these are quick lookups. You also can do this second film if you know only the civil parish, but not the townland, but your search will be much more laborious, especially if it's a large parish.) This film is the Index to the 1901 Ireland Census Districts. It is divided by country, then by civil parish, and gives specific LDS film numbers. Each parish is further divided into a number of districts (the number you got in step one above). Find the right district and you have the LDS film number to request.

STEP THREE: order the corresponding LDS film(s). These films are usually in the 800000 thru 899999 range.