Hello Poppy,
I do realise that you posted this request an awful long time ago but hopefully I will be able to answer your question.
I happened to be in Offaly last night and today at a function with my OH and decided to have a hunt for you..
I asked in our hotel and was sent to two graveyards called the Island... Finding the right one eventually after a short tour of the county

Walsh Island is a tiny village off the Tullamore to Portarlington road. It is quite small and I drove through it twice before realising I was past it !!!!!!!
I did find the old graveyard ( the new one only opened in 1953) but there sadly is no grave stones for your family I did go all around and checked them all. I went into the church attached to the graveyard and lit a candle for your reles on your behalf.
I called to the local parish priest and spoke to his housekeeper whose husband luckily looks after the graveyards and he was able to help me a lot.
Burial mounds was the norm but about 40 years ago the parish priest of the day decided to level the graveyard and grass it over with only those with headstones or markers having those replaced in the correct spot.
I took some photos for you and hope that you find them interesting. I also got the PP's telephone number (he wasn't there) and I will ring him later (I have just arrived home and still have to unpack etc) to see if he can hunt through his records and see if he can come up with any info..
Best wishes
Mo