It might be worthwhile getting a short subscription for
www.irishnewsarchive.com and doing a search for John Savage under "exact word or phrase" and for Kinsale under "all of these words". It comes up with 52 results. I'm not sure exactly how many deal with your man, but it seems the vast majority do anyway. I've seen mention of a few court cases he was involved with, his role in the Kinsale Harbour Commissioners, as a town councillor, as a bailiff of the county sherriff (was involved in several evictions), etc. I'm sure if you played around with other combinations of Savage and Kinsale you'd find more, but you'd have to pick through a lot of irrelevant stuff.
Couldn't find any references to an Isaac Savage in relation to Kinsale in the 1800s though. I wonder was the reference to Isaac Savage on the plaque erected on the Olde Bakery in 1998 an error based on the name of John Savage's son, Isaac? Maybe, maybe not.
Oldest reference I came across was 1868, a John Savage was noted as bring present at a meeting in Kinsale of electors attended by the sitting London MP for Kinsale constituency, Sir George Colthurst. Next was 1874, which refers to "John Savage, a shopkeeper who keeps a grocery and bakery establishement in Fisher-street, Kinsale".
Other bits of interest include:
Cork Examiner, 26 August 1878, page 3:
[A report about John Savage being brought in front of the Kinsale Petty Sessions for not having his daughter vaccinated under the Vaccination Act, as he claimed it made another daughter sick]:
"Dr Dorman deposed that he was the registrar of births, deaths, and marriages, and that the child Helena Savage was born on the 17 December, 1876, but, up to the present, he is not aware that the child is vaccinated."
Cork Examiner, 24 May 1880, page :
[Refers to John Savage as "the now famous anti-vaccinator, was prosecuted for the 19th time by the Board of Guardians of the Kinsale Union"]
Cork Examiner, 21 November 1899, page 1:
Higgins - On the 19th inst., at Millstreet, Eleanor, wife of Sergeant Higgins, RIC, and daughter of John Savage, Kinsale, aged 28. Funeral from Cork GS and W Railway at 11 o'clock this day by road via Belgooly to Kinsale.
Hopefully this clears up at least a little bit of the Higgins mystery! I had a look on the newspapers, found references to a Sergeant William Higgins in Millstreet, so I would think it very likely that the widowed William Higgins living with two Savages in Cork (as pointed out by Aghadowey back in 2010 on this thread) was indeed the widowed husband of Eleanor Savage of Kinsale:
www.census.nationalarchives.ie/pages/1911/Cork/Cork_No__5_Urban__part_of_/O_Connor_Ville/399681