Being of Celtic descent, and a fan of the folklore and culture, there are quite a few things that you can use in your story to spice it up. Since it is set in medieval Ireland, using local flavor and lore would add authenticity and draw the reader in. However, you will have to define many of the "Irish" words that you include. For instance, most people don't know what a shillelagh is, but it would be a common item in your story. I would also reevaluate the decision to exclude any "fantasy" from the story. Irish folklore is a strong part of the culture, and it was quite common for people to believe in leprechauns, banshees, and the mystical. In fact, it is a large part of their lives, even today. Not that I'm trying to tell you how to write your story, just trying to give you some ideas to help bring your setting to life.