When people book a ceildh band they want entertainment, so give them it.
If that means wearing matching shirts or waistcoats to look good, then its worth it if they remember you and book you again.

The band I work most with - the Family Band (http://homepages.which.net/~katriona.moulton/index.html) has a humourous approach (mostly at my expense), which seems to go down well.

But more important is the willingness to do whatever is needed, - keep playing dance tunes if the crowd want to dance, play tunes to listen to if they all need a rest, accompany the Birthday boy on his electronic bagpipes (he was good, but the invited guests will appreciate that even if he isn't), or whatever.