Shamrocks is a fun and interesting solitaire card game, that can be grouped with Cruel Solitaire or La Belle Lucie in terms of similar play. It's pretty fun and most games are winnable. The trick is that you can't have more than 3 cards in any given pile at a time. Rules for building on the piles are pretty loose. You can put a card on the top of a pile as long as it has a value that is one more than or one less than the value that is currently on the top of the pile. You can also ignore the suits of the cards when building on the piles.
What makes Shamrocks difficult
I've already mentioned that you can't have more than 3 cards in a single pile. The other challenge is that once you've removed all the cards in a pile, that pile is gone for good. You can't restart it no matter what you do. So before you remove the bottom card in any pile you really need to give it some thought. Like in many solitaire card games, you're goal is to build up four piles of cards known as the 'foundations'. Once you've built up the four foundations from Ace to King by suit, you've won the game.