Éireannach and Nightwalkers I guess could have the same kind of incompatibility backlash as humans and Nighwalkers - namely that the half-human hybrid would always be completely and utterly blind (which would also give a logical justification to why such relationships would be looked down on). Just a suggestion here.

Also, as I pointed out - long lifespan doesn't necessarily mean long period till maturity. There are Earthern beings which can hit hundred and twenty, but are full adults at three or so. And yeah ... wars, illness, predators, underabundancy of food, low conception rate, the factors which bar exponential population growth rate are many. Here I will also note that fertility period may not last for the entire adulthood - human women past forty-five or so very rarely conceive naturally despite the life expectancy being at about eighty (and lifespan cap at over hundred), for instance.