Not sure about tongas tactics. To me you have to starve ireland of possession as much you can, but you also don't want to play in your half. Sounds tough, so you will not necessarily succeed but one thing that would help is not taking threes. Kickable penalties are opportunities to hold onto the ball in irelands half, preventing them from having the ball themselves, and also allowing you more of a chance to score enough points against them to beat them.
Tonga have a pretty good team, weak in their wingers and reserve front row, could do with a better halfback. Not far off a solid 23 that could be competitive if they had a lot more cohesion and good coaches.
Their discipline was poor in that game. If they can tidy that up, they should grow into the tournament. They might be a bit better by the time they play South Africa, but they are no chance against the boks if the boks play their top team