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.