Montreal's original CFL team folded in a disastrous manner in the late 1980s. When the CFL expanded to the US, the only success out of the bunch was the Baltimore Colts / CFLers / Stallions (the story of the name is a long one by itself), but when the remainder of the US teams folded Baltimore was moved back to Montreal instead, playing once more in the cavernous and decrepit Olympic Stadium.
In spite of being a great team, anyone who follows baseball knows how much Montrealers hated the Big O / Owe / OW by this point, so no one was showing up. The CFL as a whole was struggling at this point, with no clear signs that a new national TV contract was forthcoming beyond CBC's minimal rights fees.
Well, when they made the playoffs it turned out that U2 had the stadium booked for the entire weekend. Desperate, Montreal decided to play at Molson stadium on McGill's campus, which is basically a bunch of bleachers. They sold the game out, and suddenly football was popular in Montreal again. The next year they played all their games there and sold out every one, which has continued to this day. Playoff games, ironically enough, are at Olympic Stadium - only now, they sell THOSE out too.
The rise of football's popularity in Quebec, coinciding with baseball dying a slow death, saw ratings for all CFL games shoot up, leading to a new national TSN contract that stabilised the league, and in fact it's probably healthier now than it's been at any point since the 1970s since it's a ratings powerhouse for TSN.
But TSN wouldn't have jumped on-board like it did (ratings were hovering around the anaemic level TSN drew for non-Raptors NBA games) if Quebec hadn't gotten behind the Als, and Quebec wouldn't have gotten behind the Als had the tickets become such a hot commodity, and the tickets wouldn't have become a hot commodity had U2 not booked the stadium and forced the Als to move their game.
In other words, that's how U2 saved the CFL. And now you know.