It's almost impossible to give a definite answer to this from just the photo. Ideally you need to check in either a transcription of the memorials made before 1980 (the earliest date on the added stone), or the burial register, in the hope that one or both of those will identify who was buried in that plot and when. Another possibility might be to see if you can contact whoever was responsible for putting the extra stone there, as they may know what was there previously - and they are presumably related to you.
I was recently involved in arranging an extra inscription on a memorial. The masons said it was more or less impossible for them to work on the existing stone while it was in place, so we decided on an extra tablet to be carved off-site and placed at the foot of the existing one. I wonder if something similar might have happened here. The original inscription might have eroded, or possibly when Denis and Bridget died there wasn't room to add their details, so it was decided to cover up the original with this tablet, giving only an abbreviated version of what was there before.
One more thought: it may be clear to you since you know your family, but is there any possibility that the Denis and Bridget whose details are given at the bottom are also the parents mentioned at the top? If so, could they perhaps have had two children buried there initially (Nellie and an unnamed son), but when they themselves died there wasn't room to add their details on the same stone, so this extra one was made?
Finally, if an inscription says "In Memory Of" you can't necessarily deduce that ther person is buried there - I've come across memorials in England mentioning family members who had emigrated to America and were buried there; and one of my ancestors is mentioned on two different stones in the same cemetery. Only if it says something like "Here Lies" can you be absolutely certain that that is their final resting place.
Arthur
