Dublin layover

What time do you arrive?

If you really want to do this, forget the bus. It will take too long (the trip itself plus waiting for the bus that only runs every half hour.) You will need to splurge on a taxi.

You'll have to walk from your arrival gate and go through immigration and customs. Then walk out of the terminal and get into a taxi. The taxi shouldn't take more than 30 minutes unless it's rush hour. (I've arrived in Dublin and have had to wait for a taxi. The line with people waiting for a taxi was longer than the line of taxis.)

On return give yourself a good three hours to get back, check in, go through security, etc.

And what are you going to do with your luggage?

I get in at 8:45 am on a Thursday. My outgoing flight is scheduled for 13:40 pm. I'll just have a 30L carryon that I can bring with me in the cab.

Regarding having a pint, I don't think it's until late morning you'd typically find a pub even open, let alone with any kind of lively atmosphere. I'm sure it's possible but you might actually enjoy it more at the airport, where you can certainly get one or two at lunch time without worrying about getting back to catch your flight. Enjoyed a Guinness myself recently while transiting through there en route from London to Dulles.

When you land in Dublin passport control is at the end of the hallway. Turn left and you go through the connecting flights corridor. Go straight and you see the immigration line. You'll be able to get a good idea then whether it will be worth it to try to get into Dublin for an hour. Just as an FYI, it's taken me as little as 20 minutes but as much as 60 minutes. It all depends on how many flights have come in.

For anyone else reading this buses are far more often than every half hour. There are three city routes, each every 20 minutes, then the long distance services of three different companies from Belfast, and various other routes from other points in both Northern and Southern Ireland.
All the routes, city and long distance, take about 30 minutes.
But yes time is of the essence, although how much time a taxi will save is debatable.

If it was me, I'd do what Slate suggested and grab a Guiness at the airport.

Even if you got through immigration quickly, you might hit the tail end of rush hour traffic.

If you really want to go into town for a pint, pubs cannot legally start serving alcohol until 10:30 AM. There are a few who are licensed to serve as early as 7 AM. Unfortunately, only a handful remain in Dublin. You need to find those if you want an early morning pint. You can't just pop into any pub.

There are plenty of buses into Dublin from the airport. They run every few minutes. But they don't all go to the same place or travel the same route. If you are going someplace specific you might find they only run every half hour.

I've been to Dublin numerous times and mostly took buses into town. (Now I take taxis as the routings have changed since the pandemic.) Sometimes the bus I was on would wait an extra five or ten minutes to fill as many seats as possible before departing the airport.

Thanks everyone. This has been helpful. I will play it safe and have a Guinness at the airport. My excitement for seeing Dublin is a pretty sure sign that should be its own trip in the near future.