In a large skillet with a heavy bottom (I use a heavy copper skillet), add the butter and olive oil. Turn the heat to medium, and let the butter melt.
Meanwhile, peel, halve, and slice the onions into half-moon shapes.
Add all of the onions to the skillet, and cook over medium-low heat for 45 minutes, stirring every 5 minutes.
The onions are done when they are a light golden brown.
Pour in the whiskey, and stir to scrape the brown bits from the bottom of the pan. Let the whiskey cook until its reduced by half.
In a serving bowl, add the cream cheese. Pour the hot caramelized onions on top, and stir until it melts completely.
Finally, stir in the mayo, salt, pepper, and Dubliner® cheese.
You can serve it as-is, but I like to pulse it in a food processor to make it even creamier.
Serve with pumpernickel toasts.