So this is something that has caused me no small amount of grief, and I thought for posterity's sake I would post screenshots of where I, personally, have been able to find the nests of Fireball, Ironbelly and Vipertooth dragons on the Island. I have wyverns and Ridgeback dragons disabled on my server, but if anyone else would like to add to this list I and many others would surely appreciate it!

Only one rule for the screenshot, make sure you have your map open with 'show player location' turned on so others know where to go.

Fireball Dragons: On Dead Island's beach, next to a medium-sized rock. Almost in the water.
https://i.imgur.com/PH0ypdI.jpg

Ironbelly Dragons: In the Western Coast (mod main page has them incorrectly labeled as spawning in Western Plains), on a cliffside overlooking the sandbar below where the dragon tends to prowl.
https://i.imgur.com/uoKnZlU.jpg

Vipertooth Dragons: In the forest surrounding Red Peak, on a small protruding cliff surrounded by trees. Hard to see with graphics set to High or better, due to the darkness of the forest.
https://i.imgur.com/s5OqPxz.jpg