Unfortunately, there is currently no café within the National Museum of Ireland - Natural History.
However, as the Museum is in a central location, there are plenty of eating options nearby: Baggot St, Nassau St, and Grafton Street are only 5 - 10 minutes walk away.
You might like to try the family-friendly cafe at NMI -Archaeology, which 5 minutes walk away on Kildare Street, and squeeze in a trip to the Vikings, Bog Bodies, Egyptians, and much more whilst you're there!
Walking directions from Natural History (Merrion Street) to Archaeology (Kildare Street)
Or why not bring a picnic and enjoy your lunch in nearby Merrion Square or St Stephens Green?