Maybe because mosquitoes don't have a culture? The trauma of the famine spreads across generations, it can have changed the mentality of the Irish people durably, made them have fewer children on average.
But as something that happened everywhere[1], it's also hard to attribute to the famine. Then again, the only source I found (I didn't look hard) for Irish fertility data went back to 1975, so it might be hard to evaluate the theory.
[1] Demographic transition has not happened everywhere
It could have, but AFAIK we aren't seeing similar effects for other nations after having great famines with large mortality, so I'd assume that it's not the case.
But as something that happened everywhere[1], it's also hard to attribute to the famine. Then again, the only source I found (I didn't look hard) for Irish fertility data went back to 1975, so it might be hard to evaluate the theory.
[1] Demographic transition has not happened everywhere