Should i tell my family before or after?

my boyfriend and i are planning a trip to ireland in the fall and decided we should get married while we are there, so we can do it in a castle. i generally hate weddings and have never wanted to have one so i thought this would be a good option (just the two of us and the marrying-guy). i wonder though if we have to tell our families what we plan before we go or is it okay to mention it when we return? its no big deal to either of us and we don't want gifts or a reception or anything, but i don't want my mum or his mom crying over this at any point. should we maybe just send them a postcard with the news?