Why is it tasarLAR? Is this an irregular verb?
The verb root is "tasarlamak" :) Ergo, "tasrala+R" = "tasarlar." The plural of this would be tasarlarlar.
What about elbiseleri or elbiseler ?
no, that doesn't work in Turkish, general (i.e. indefinite) objectss are almost always in singular https://www.duolingo.com/comment/7736911