it should be open until 9pm at least on the Sunday of St Patricks Day weekend. We were there last year at that time, to be honest I didn't find it got very quiet in the park on either day, Saturday or Sunday, until after 8pm (BTM still had 60-minute lines after 8pm on both evenings, as we had fastpass we'd got earlier for BTM on both nights and so could see the regular line queues as we approached the ride).
We were in Fantasyland and Frontierland from around 7pm through to 8.30pm on the Sunday, there were lines for just about everything that was open I'm afraid, until well after 7.30pm in both lands. The weather was very mild on the Sunday, maybe that made a difference?