Heat olive oil in a small skillet over medium heat.

Add mushrooms, sliced onion, garlic, and Guinness; heat, stirring, until all liquid has been absorbed and vegetables become tender and caramelized.

In a separate skillet, melt 1 Tbsp. butter over medium-low heat.

Add 2 pieces bread and top with Dubliner cheese and bacon.

Grill until bread is golden brown; remove from skillet and add 1 Tbsp. butter.

Add remaining 2 pieces bread and divide mushroom mixture evenly between the two; top with previously grilled bread/cheese/bacon.

Grill until cheese is fully melted and both pieces of bread are golden brown.