Hi OP,
I am from Mayo (left to go to Uni and never returned to live there) and got married there many years ago. A friend got married in Knockranny and it was lovely.
Can I just say though, that Westport, while lovely, is very touristy.
Now, I am not sure how many people are planning on visiting but there is an amazing place (I have no connection with here, see my posting history, I live in Dublin and do not work in the hotel industry) in Ballina called Mount Falcon, which has an old lodge hotel and a lot of self catering lodges on site where guests can stay. The lodges are 5 star (I've stayed there multiple times) and the whole place is fabulous, private lake, long woodland walks, swimming pool and spa. It may be too costly, but if I was taking my family to Ireland for a dream wedding, this would be the type of place I would choose.
Also, weddings in Ireland cost a fortune but I did mine on a budget, as DH and I paid for most of it ourselves, and we did the flowers ourselves (my darling aunt got loads from a florist and herself and her best friend did all the arrangements); we had none of the add ons, and we had a fabulous day. Most modern brides that I know (who married in the past 5 years) are doing more "low key, home-made, tasteful, no add ons" events now, and I think I would go down this route if I were your DD.
I hope you have a fabulous day wherever your DD chooses - and if you let us all know on here, we can place the "Child of Prague" outside our back door for you so that she gets good weather!!!!!