A bottle opener that can be disguised as a sgian dubh.
Whether you are attending a wedding or other event, many venues these days get all bent out of shape about weapons’ so you may not be able to convince them that your sgian dubh is not a knife. On these occasions you need a sgian bru.

Besides, when drinking is involved, which is more useful? A knife, or a bottle opener? (Some would say a knife, but then you don't want to be at that party.)