Diarmuid himself is obviously not fit for an infiltration attempt because he's just a kid (even if he is training to be a knight). However, he would be good for any kind of support that they might need before or after the attempt. If there is some kind of safe house location for meeting, he can help set up food or supplies. He is also good for running messages between people, since who is going to suspect a kid? His regain was a magical bracelet that slightly increases his strength and will deflect the first shot/blow someone hits him with, plus he can dual-wield swords, so it's not like he is completely defenseless!

One thing that might help a little more directly with the infiltration is his cloak of invisibility. It is actually his father's, so it will hide a full-sized adult from almost all means of detection (magical, physical etc). Though it is magical in nature, the person using it doesn't have to be magically inclined to activate the ability. It is activated just by raising the hood.

If anyone is interested in using it, feel free to let me know!