I tried a dozen different pairs of dates, all of which worked fine as round trips, showed no flights available using the Multi City option, and yes, I DO know how to book multi city trips, I've done it dozens of times with other airlines.

As for different sites for different locations, what I see is on https://www.aerlingus.com/html/en-US/home.html - showing specifically that it is a US version of the site. The last time we flew EI, checking in for the return flight from the Netherlands, we saw a very different version of the site than we see from the US.