I'm not sure on pressure cooker, since tarhonya (as rice) doesn't take that much time to cook and I usually use pressure cooker on food that needs quite a long time (beef, pork, goulash, ...) when I need to cut this time.
So, how I prepare it, is quite simple - and I must admit I really like tarhonya. You need to have the tarhonya:water ratio 2:3. You need to dry roast tarhonya in hot pot until it gets nice brown (it's pasta really, so when raw it should be very fair beige yellow-y) - just be patient and stir ocassionaly. When the colour is nice brown, add oil (i'd use as many tablespoons of oil as I used cups or tarhonya) and salt, then add the water and put the lid on. When the water boils, lower the temperature and let it cook for about 10-15 mins. You should clearly see little cavities on the surface of tarhonya, that's where the water evaporated through. Take the pot off of the stove, put in on or wrap it in a kitchen towel and let sit for another 20 minutes (without opening the lid).
You should by now have beautiful, brown-y, moist side dish to meat or vegetables or anything, really.
So you can see that use of pressure cooker would be really unncessary and maybe even impossible - but if you discover some way, let me know!