It's a good while since I was in Westport but when there, I ate in a French restaurant called The Lemon Peel. Don't know if it's still there but there was about 20 of us.
There are a couple of others that I can't think of the names but maybe the Westport tourism website might throw up a few.
I second the Shebeen, nice steaks in front of a cosy fire. Good friendly place. I have had steak there several times, but never tried anything else so I can't vouch for the rest of the food.
There's a good place in Louisburg, coming from Westport it's down the main street on the left, but I can't remember the name of it, Good food, room for a crowd and reasonable.
quay cottage was class but now changed owners so have taken it to a more run of the mill angle......evening offers etc etc which in this climate is understandable.
Mango's is excellent.. but its not a huge space so a group of 20 might push it. JJ O'Malleys used to be very good but its gone down a lot.. it has a HUGE menu which is always something to be wary of. I ate there recently and the potatoes and veg seemed old and microwaved.
Quay Cottage has also come under new management so its not as good as it once was.. but its still not terrible. The restaurant in Knockranny hotel would be a good choice and is in a lovely setting.