Preheat oven to 325 degrees. Pat ribs dry with a paper towel, then season generously with salt and black pepper.
Heat half the oil over medium-high heat in a Dutch oven. Working in batches if necessary, sear the ribs well on all sides then transfer to a plate.
Remove the scorched oil from the Dutch oven and replace it with the rest of the fresh oil being careful to not remove any fond. Return the Dutch oven to medium-high heat.
Add the onion, celery and carrot, reduce heat to medium-low and cook for 3 to 5 minutes or until beginning to soften.
Add the garlic and cook briefly--about 15 seconds.
Add the flour and cook 1-2 minutes, stirring constantly.
Add the tomato paste and cook briefly--about 15 seconds.
Reduce heat to low. Slowly add the beer then bring to a boil, scraping up any browned bits.
Add the beef broth, thyme sprigs and the ribs back to the pot. Bring to a simmer over high heat but do not boil.
Place aluminum foil or parchment paper over the pot first (to help minimize evaporation) then cover securely with the lid. Transfer to the oven and cook 2 1/2 to 3 hours.
Skim off as much fat from the top as possible. Season to taste with salt and black pepper. Serve with colcannon.