As others have said, you'll probably be there right at the start of the Spring Break season. Easter is late in 2017 (April 16) and while not all school districts base their Spring Breaks on Easter a fair number still do (e.g., LAUSD). There is also the Natural Product Expo at the Anaheim Convention Center (March 10-12, 2017), which means the area hotels might be impacted. Conventions, in general, do not necessarily mean large crowds in the parks, but Natural Product Expo is very large and it's been mentioned in the past as one that really impacts area hotels. So it's something to keep in mind as you make hotel/motel reservations. Lastly, as mentioned, there is also usually a large number of Cheer Competition in March. There impact is variable, but most guests have reported noticing cheer groups in the parks during competition/workshop weekends. There is a chance that the first part of your trip is before DLR goes to their Spring Break season schedule, which may mean shorter park hours and reduced entertainment (on weekdays). It's too early to know at this point what the schedules will be (the operating calendar is posted 45-days/6-weeks in advance).
Paint the Night (and
Disneyland Forever and WoC: Celebrate) debuted as part of the 60th Anniversary Celebration. Given Disney's track record with successful/popular parades/shows, I be stunned if PtN ends with the end of the 60th Celebration. Disney usually presents the same parade for years (e.g., Soundsational debuted in 2011 and it's still being presented). So, I'm hopeful that PtN and Disneyland Forever will be presented beyond the end of the 60th Celebration.
In terms of St. Patrick's Day...I was there this year and the parks themselves do not do much. There are a few treats (e.g., candies, caramel apples, marshmallow wands) available inside the parks and some merchandise (e.g., plush Mickey, pin), but that was about it. Some of the restaurants in DTD had special St. Patrick's Day menus (some for only that day, others for the week). The
Parks Blog had a post announcing the DTD offerings a few days in advance. There were a lot of guests wearing green in the parks on St. Patrick's Day (and some festive hats), but that was about it.