Luvdaislands, that would be a challenge if you are relying on public transportation. Renting a car will eliminate longer road time and transfers. But if using public transit, here is a possible plan if you land in Montego Bay, fly out of Kingston.

Montego Bay to Negril (Knutsford or JUTA)
Negril to Ocho Rios (Knutsford or JUTA)
Ocho Rios to Portland (private driver or public transit)
Portland to Kingston (private driver or public transit)

Unfortunately, as Sammy said, Portland isn’t on Knutsford’s regular schedule yet. They only go to Portland when special events are held there during the holidays. However, some of the public buses traveling between Ocho Rios-Portland-Kingston are nice. Huge with AC and I think around $12-$15 depending on destination.