As of yesterday afternoon there were still (quite a few?) tickets left for the opening show by Macnas on Saturday. It's a free show but judging by last year it'll be excellent, one of the best you'll see this year. So get on down to that boxoffice in Filmbase in Temple Bar and grab a ticket or five.
Some of the shows in smaller venues seem to be getting close to capacity. Personally I like to pick and choose when the festival has started - there are so many shows - but if there's something you're specifically looking to see, again I'd suggest booking now.
Man of Valour seems to be one of the best sellers. This isn't its first run, and it's previously received strongly positive reviews. It's on in the Samuel Beckett Theatre, previewing on Thursday 8th. (Even though the festival starts on 10th September, several of the shows preview before then.)