Jewels, my mother's father was one of those who suffered with a mustard gas attack during WW1 though survived.
I was able to trace my father's ancestry a few generations and found that an ancestor came to this country from County Meath during the Irish potato famine. Couldn't go back any further for the same reason as your mother in law found. There was a catastrophic fire at the Irish records office in 1922 where all the records were destroyed. Efforts have been made and are on-going to try to re-create past records - a very lengthy process such as going through churches parish records. Back in the 1830s/40s the literacy rate in Ireland was 30% so although priests were literate asking people how they spelled their names would have resulted in errors in the records. Although I was able to see that ancestor's name in the British census records for 1851, I've not been able to find that name in Irish records. It's quite possible that he's in the records but with his surname spelled differently to what he used for the British census.