Ice crystals in my jameson?

It means someone watered it down.

Pure ethanol freezes at -114°C (-173°F), and no domestic freezer will ever get that cold.

But Jameson - like all alcoholic drinks - is a mixture of water and ethanol. Water obviously freezes at 0°C (32°F), and the water molecules in the whiskey will start to clump together and form ice crystals when the bottle gets to that temperature. I believe Jameson is 80 proof in American terms, or 40% ethanol in the more rational measure of alcohol content used in other parts of the world. Our freezer will get down to -25°C, and that would be low enough to end up with a bottle of 40% whiskey that looks like it's frozen solid. It wouldn't really all be frozen, though; it's actually a sort of slushy with molecules of still liquid ethanol distributed in a matrix of water ice.

If you were to take a sip of the liquid that remained in your partially frozen bottle, I bet you'd find that it has a helluva kick. But I doubt if it would taste much like Jameson since some of the flavouring components might be locked in the ice and others with a higher freezing point will be more concentrated in the liquid.

Fractional freezing is a technique used to produce some high alcohol-content drinks: the fermented liquid is chilled to the point where water forms ice and then the ice crystals are filtered off. I've never tried it, so I don't know how practical it is and I doubt if it would be very efficient, but if you were to want a very high ethanol-content drink, you can supposedly put a bottle of cheap vodka upright in a freezer, and check periodically for the formation of ice crystals. When ice starts to form, you periodically spin the bottle so the crystals are pushed to the walls of the bottle by centrifugal force. If you get this right, you supposedly end up with a hollow cylinder of ice surrounding a core of liquid that has a much higher alcohol content than the original. (But you'll never get to 100% ethanol for complicated chemical reasons.)

I bet our resident Russian will be along shortly to tell us all exactly how to carry out this procedure in the most efficient way possible. 😁

Whisky can't be frozen. (At least not by the freezer in your kitchen).

Someone has put something in it - most likely water. Either that or your freezer is insanely cold, but I doubt this is what's happened.

Never. Great choice by the way.

Thought this was going to be about adding meth to Jameson. That's definitely not normal...

...but on meth it is.