IR is a sub specialty of Rad, but I wouldn't call Rad Onc a subspecialty of Rad. It's a separate residency.
Rad Onc, you work with cancer patients, so it's mostly people who are middle-aged and older. It can be emotional. In private practice, you spend time in 1hr consults with new patients. Then you work on a computer to plan their treatment with a dosimetrist and medical physicist. You have 15min checkups with them during their 4-8 wk treatment to manage side effects.
It's a great specialty for introverts, because there's lots of computer time, and lots of intense face to face time where you're talking about serious, life changing diagnoses. Anatomy and physics are important. You spend a ton of time looking at MRIs and other scans and contouring the tumor area and determining the dosage.
Patients don't come to you until they have a diagnosis and someone else, their primary care doc or the surgeon or med onc doc who saw them before you, decides they're a good candidate for radiation therapy. You just focus on treatment. You can treat almost anything, and make a big difference in quality of life, but you can't cure everything. Rad Onc obviously focuses on the tumors that are viable targets of radiation. You usually don't order diagnostic tests.
You can also do brachytherapy if you want procedures.
IR is more about radiology-guided procedures and radiation medicine and tracers, from what I hear.