Melt the butter and mini marshmallows in a large saucepan over medium-low heat, stirring frequently. Remove from heat when the marshmallows are almost melted and continue stirring until smooth. (See notes for microwave instructions.)
If using Bailey's, stir in now.
Stir in the Lucky Charms.
Gently spread the cereal bar mixture into the pan.
Let set completely, then cut into squares and enjoy!
Notes
To melt the butter and marshmallows in the microwave, use a large microwave-safe bowl and heat for 45 seconds, then stir. Continue heating in 15 second increments, stirring after each, until melted and smooth.If using homemade marshmallows, instead of letting the marshmallows set, stir the cereal directly into the bowl after beating the marshmallow, then pour the mixture into the prepared pan.Lightly press the marshmallow cereal mixture into the pan. If the mixture is packed in, the cereal bars will be hard.This recipe can be made in two 8x8 pans. For thicker cereal bars, use a single 8x8 pan.