Croeso. Plurals are a bit tricky in Welsh - endings can be added, taken away or swapped for another, and sometimes the vowels just change instead. By far the most common way is adding -(i)au though.
It's good if you're ahead of the game as long as you don't get carried away ;)
You're right, most verbs end in -(i)o but cinio's just a random noun thrown in to confuse you! The verbnoun "to dine" is ciniawa.
Another similar one that throws people is the noun croeso "a welcome", which looks like a verbnoun too. "To welcome" is actually croesawu.
Edit: I should also say, we don't used ciniawa much. It sounds pretty formal. Usually just say bwyta cinio "eat lunch".
You can think of the tricky "ll" /ɬ/ sound as of an unvoiced "l". This sound is rare in European languages, being present only in Welsh, Icelandic and Faroese. https://en.wikipedia.org/wiki/Voiceless_dental_and_alveolar_lateral_fricatives