Hi Pammi,
I thought you would be a bit shocked by this information - I certainly was.
Did you know Patrick had changed his name and your grandfather was registered as Hamilton?
Also that Catherine Mary Cahill was Mary Burke?
All this throws doubt on you having a great uncle William O’Brien - at least not from Limerick so not the one you have in your records.
Your great grandfather’s army record apparently gives his birthplace as Ballyragget, Kilkenny.