I still think the q-ship concept is the way to go on this. Make it look like a big, fat, attractive target. Concealed weapons "pods" if you will. Let the bastards get in range of our weapons, confirm their intentions, then give them one helluva surprise.
Or go with a big ship and RIB/Fast Attack concept. Something with a big enough main gun to put the pirate's motherships on the bottom, but I don't like the idea of being in a similar sized and unarmored vessel to the pirate attack boats. Because if it's a small boat, they just need to get lucky with an RPG or small arms fire to really ruin your day.