As mentioned above the main bus station in Cavan is about 10 minutes walk towards town from Breffni Park. As you say, you will be cutting it tight to see all the game and catch the 8.40pm bus back. That said, the bus will have to pass by the gates of Breffni Park, so its quite likely to be delayed anyway for a period after the game. My guess is that if you leave Breffni Park anytime before 8.35pm and walk towards town, you will meet the bus (probably stalled in traffic) along your route. Flag down the bus, smile at the driver and hop on.