In a large mixing bowl, combine the pork blood, chopped fatback, steel-cut oats, breadcrumbs, salt, black pepper, nutmeg, cloves, allspice, dried thyme, dried sage, and chopped onion. Mix well until all the ingredients are evenly combined.

Prepare casings for stuffing the pudding. If you can find natural sausage casings, soak them in water according to the package instructions. Alternatively, you can use synthetic casings or shape the pudding mixture into patties.

If using casings, attach a sausage stuffer to a sausage maker or use a piping bag with a wide nozzle. Fill the casing or piping bag with the pudding mixture, taking care to remove any air bubbles.

If using casings, tie off the ends of the filled casings to create individual sausages. If shaping into patties, simply form the mixture into desired shapes.

In a large pot, bring water to a gentle simmer. Carefully add the pudding to the pot and cook for about 1 hour. Ensure the water is not boiling vigorously to prevent the casings from bursting.

Once cooked, remove the pudding from the pot and let it cool slightly. If using casings, you can remove them before serving or leave them intact.

Slice the pudding into thick rounds and serve as part of a traditional Irish breakfast or use it in other dishes as desired.