I have entered a sprint triathlon on a whim and have a 29er mountain bike. Is there a road suitable tire for a 29x2.25 wheel out there? Looking for something that will give me good roll and as little resistance for my 110kg as possible.
Of course. 29 inches is the same rim size as 700c, so there are plenty of tires out there.
For maximum speed, you want the narrowest tire that you can safely get on there. Take off your tire and measure the inner rim width. Refer to the chart at the bottom of this page to see which tire widths would be suitable.
Yes, there are tires out there that are smooth and will fit that rim.
You'll probably find that tires marketed for comfort hybrids, cruisers, touring or "urban/trail" bikes are were you'll find a big enough smooth tire.
Schwalbe makes the Super Moto in 60-622, which is specifically marketed as a competition-appropriate smooth tire for a 29er.
If you're willing to spend a lot more, consider replacing the wheels with Cyclocross wheels, some other 622mm wheel with a slightly narrower rim than a 29er usually comes with. That would allow you to get into the wide end of regular road tires where there's a ton more choices made for competition on paved surfaces.
Note: smaller tires will change the handling and lower the bottom bracket closer to the ground, so don't get too crazy.