I’ve been wondering what the correct plural for those little facial expression images is. Lots of people write it ‘smilies’ which would make the singular ‘smily’, right? (As -y usually changes to -ie- in plurals.) Smily has the suffix -y which often makes the word adjective (as the base word here is ‘smile’). However, ‘smily’ doesn’t seem to be a word even; ‘smiley’ is used as the adjective, too.

If the base word is smiley, it should be made plural by just adding -s at the end (compare: key–keys, not kies). There is a grammar rule “-y after consonants is changed to -ie-, but -y after vowels doesn’t change”.

QED