I note he served in the 74th (Dublin) Company, 8th Battalion Imperial Yeomanry, yet your message on the Anglo-Boer web site gives his place of birth as Co Antrim and the 1911 census appears to place the family at 61 Unity Street, Court, Co Antrim. That may not be unusual but I must admit I was a surprised to see he lived in the geographical north of the Island of Ireland rather than the south (not to be confused with Eire and Northern Ireland as of today); perhaps the “Belfast” companies were over subscribed.
Although many Imperial Yeomanry men re-enlisted for a second term, they only enlisted for one years service, so be aware that the 74th Company was raised in 1900 and originally part of the 16th Battalion, it was only transferred to the 8th battalion I.Y. in 1902, but I don’t think that should affect his record.
As to his death registration, as he died well after civil registration began in the whole of Ireland, even if he died in a mental institution, TB sanatorium or other hospital, even prison, his death would still be registered within the Civil Registration system. However, if he was admitted to hospital unconscious or otherwise debilitated he may not be recorded under his full name, his death could be registered as “unknown male” or “male Marnell” and of course his surname may have a corrupted spelling.
Hospital records are rare and you would have to know the name, at present I don't think you can even confirm he died in Co Antrim.