I haven't found the death of John Leslie Montgomery Connor yet either but here's a bit more which might be of interest-
John Leslie Montgomery Connor (d.1896) m. Elizabeth Keightley?
Leinster Leader (Naas), 28 July 1894: … regret to announce the death of Dr Willis L. Connor, __, of Liverpool, which took place at the residence of his brother Mr Leslie M. Connor, C.E., skilful, eager and prompt in the discharge of his duties, he soon after graduating Arts and Medicine, secured…
Willis Lewis Connor d.1894 age 44 (informant not brother!)
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1894/05957/4693016.pdf 1. Son? (1886)
Cork Constitution, 21 Oct.1886: Connor- October at Woodstock, Athy, the wife of Leslie M. Connor, B.A…., of…
2. Daughter (1887)
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1887/02536/1939344.pdf 3.
Leslie Montgomery Connor (1889-1890)
Leinster Leader (Naas), 29 June 1889: Connor —June 24th, at Duke-street, Athy, the wife of Leslie Connor, B.A., C.E., of son.
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1889/02462/1914636.pdf (O’Connor)
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1890/06088/4737073.pdf4.
Elizabeth Ethel Frances Connor (1891)
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1891/02377/1887164.pdf (O’Connor)
Leinster Leader (Naas), 29 Aug.1891: … at William-street, the wife of Leslie M. Connor, B.A.. … C.E., of a daughter.
5.
Margaret Mary Connor (1894)
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1894/02259/1849549.pdf (O’Connor)
Leinster Leader (Naas), 26 May 1894: Connor, May 12th, at William-street, Athy, the wife of Leslie Connor, B.A. …C.E., of a daughter.
6.
Annie Josephine Connor (1895)
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1895/02203/1832438.pdf (O’Connor)
I checked for the last three girls in 1901 and 1911, vital records, etc. but no luck finding them. At the time I didn't know mother had remarried and moved away from Athy.