Just realised that I made a silly mistake in saying that the Military Archives in Cathal Brugha has records from the War of Independence. It only has records from the Civil War! 10 big boxes of them! However, they do have written statements that were taken from some of the veterans of the War of Independence during the 1950's. These statements were mostly taken from local leaders and commanders though.
The National Archives in Kew, London has the records on the War of Independence. Once again, they are not very well indexed so expect to spend quite a bit of time there. They have books detailing the sentences handed down during court martials, so if your granduncle was actually sentenced, then he would probably be included there. However, most prisoners were simply interned without trial.
I think that the only internment camp whose records still survive at Kew was the Curragh. I have photos of some of those records but the only Nevilles that are listed were three from Tullamore, Co. Offaly. However, the records I have are from the second half of 1921 so it may be that he had been there at an earlier date.
