While the hobbits costumes range from our history’s 1700s – 1800s-style clothing, the LOTR movies depicted a very medieval Saxon style for the culture of Rohan, mixed with lots of Celtic knotwork. It was absolutely stunning. And note that Eowyn only wore fancy clothing while at home, but for the walking trip she had to make to Helm’s Deep, she wore a practical, simple outfit: a simple gown with a simple over-tunic. She looked beautiful. The gown was of a shorter length, perfect for walking. The fabric was rougher, fine for being exposed to the elements and keeping her warm. A key to this simple medieval style, for men and women, is layering. Wear a long-sleeve tunic or gown, and top that with a shorter-sleeved tunic or gown (the length of the tunic/gown itself can be shorter, too), and top that with a sleeveless surcoat or tunic, or a medieval hood, and/or a cloak. Be sure to wear comfortable boots or sandals—no modern tennis shoes!
You might also want to add the distinctive celtic horse symbol if you can, anywhere you can. The Rohan style had horses everywhere.
Here are some sites to help visualize and make the clothing style of Rohan: