Searching for a Doyle

This is a picture (not in great condition) of someone who I believe to have been one of two people. Either it is John Doyle, born August 1864 in Killorglin, Co Kerry Ireland, or his younger brother Denis Doyle, born January 1869 in Killorglin, Co Kerry, Ireland.

It seems that John emigrated to to the US and lived there for the rest of his life, while it is possible that Denis only stayed in the US for a period of time before moving on to Australia. They had a sister Julia, born July 1866, who also emigrated to the US, married in Rhode Island and lived the rest of her life in Connecticut. I am not sure whether that might indicate that either John or Denis served in a state militia from this north-eastern area of the US?

The brothers' dates of birth indicate that this photo dates from the post-civil war period. I am informed by knowledgable participants in another forum on this site that the uniform may well be that of a state militia around the time of the Spanish American war. Which regiment it might be is unclear.

Are there any records available showing who served in these state militias, which show more information than just the name of the individual and his dates of enrolment?

I would be most grateful for any suggestions.