People often wonder why Irish Dancers tan their legs for competition. To be honest there isn't really a clear answer to this.
Some say it shows off our muscle tone, like in body building competitions. In body building, one uses tanner to the extreme to showcase their muscle definition. Other says Irish Dancers tan their legs for a better stage look. The lights on stages, especially the majors, are very bright; the stages also have backdrops usually with bright colors. The tanned legs are thought to make one not looked washed out while on stage.
Nowadays if you don't tan your legs at a major competition, you'll stand out. I couldn't tell you if it would be in a good or bad way. However, I see both sides. It may make judges notice you, but I hope you practiced if you want that extra attention ! haha :) On the other hand, you may look out of place. Like at the Worlds, everyone tans their legs. If one were to show up with untanned legs then they may look inexperienced and judges may not watch you.
Irish Dance is a subjective sport so it's up to dancer, teachers, and family whether or not tanning is for them. My personal preference was to only tan at the majors and not small competitions. Also, younger age groups are not allowed to tan, wear rhinestones on their dresses, and makeup; this is to keep them looking natural and not like Toddlers and Tiaras.