There are 2 excellent Italian restaurants in Salthill, both owned by the same family, called Da Robertas. They generally need to be booked well in advance.
There is also one* on Quay Street but I don't remember the name of it.
For a group of 20 women, I suggest Kirby's on Cross Street. It can cater for big gangs and as it is attached to a superpub (Busker Brownes), they won't kick them out until 2am. It's not Italian though.
*I don't recall the name but if nobody posts it here, I will walk by it on Saturday and post it here