What abilities can I use to protect my team from a Junkrat ultimate?
Solution 1:
Lúcio's Sound Barrier, Mei's Ice Wall, and Zarya's Projected Barrier can be used to protect other teammates. It's not quite the same thing, but judicious use of Mercy's Guardian Angel (which can target nearby corpses) and Rez will also counter the RIP-Tire (but not prevent it from killing people in the first place).
Reinhardt's Barrier Field and Winston's Barrier Projector will protect anyone on the appropriate side. However, the RIP-Tire can move through these shields, so they are not as reliable as the previous methods. In particular, Reinhardt's Barrier Field is a poor choice, since the enemy Junkrat merely has to hold off detonating it until it's literally on top of your Reinhardt to guarantee a kill on him.
Solution 2:
The easiest, most readily available way of completely nullifying a RIP-tire is Zarya's barriers. Even though it can only protect two players - Zarya herself and a teammate - it is guaranteed to save both of them if the detonation goes off whilst protected by the bubbles. Zarya's barries can absorb up to 200 damage, but will always fully absorb at least one hit, even powerful ones like the RIP-tire's 600 damage and Self-Destruct's 1000 damage. Of course, this is limited to only two players, but "baiting" an early explosion by running out in the open and shielding at the last moment is sometimes enough to waste a RIP-tire completely.
Lucio's Sound Barrier has to be perfectly timed if your team is somewhat damaged, but it can protect the entire team, and it's enough to save even a Tracer caught point-blank.
For the more unreliable method, Mei can step in front of her team, use her Wall to separate herself from the others, again baiting Junkrat into exploding prematurely, and save herself with her Cryo-Stasis. Using the Wall to completely block off a RIP-tire sounds like a good idea in theory, but the tire can simply climb up the wall, look for other entry points, or even wait out the duration. The RIP-tire has a lifetime of 10 seconds, which is usually enough for a wall to vanish if the tire was deployed from nearby.
Though as always, the best counter to a 5-man wipe is a 5-man Resurrect ;)