Ryanair offer travel insurance policies as an add on to a flight booking, or they can also be purchased as stand-alone products. Ryanair travel insurance is offered in two levels, so you can choose the option that's right for you and your wallet.
Jamaica's rainy season coincides with ours here in Florida (I am in the Tampa area) for the most part. We have anywhere from 30-50% chance of rain daily until hurricane season is over. I didn't mean you have rain every day, all day from May to December in Jamaica, but I assume it is a lot like here in Florida.
The rainy season lasts from June to December. November is the wettest month, but showers occur frequently during the other months. Grenada lies south of the usual track of hurricanes, but when they do occur, as in 1955, 1979, and 1980, they often cause extensive damage.