There are a couple possibilities for what is causing the skin irritation in your case.
The first is simply skin breakdown from moisture and friction. In this case, keep the skin clean and dry is the most important step. However, you will probably also have luck if you apply a thick barrier moisturizer to the area. Hydrolated petrolatum, or some other "greasy" ointment, would be your best bet, as this stays in place for a long time and keeps out moisture better than other non greasy lotions and creams.
Sometimes fungal infections can be the cause of the redness and irritation. Fungal irritation is usually caused by yeast organisms ("thrush") and it tends to be more red and excoriated than simple skin irritation, often with small 'satellite' areas of spreading red spots. Fungal infections in these areas can usually be easily treated with an over the counter antifungal cream, in addition to good skin hygiene, although sometimes stubborn cases will require a stronger cream or a prescription oral antifungal medication.
I suggest talking with your primary care doctor
, who can help you figure out what exactly is going on and help you determine if a prescription medication is necessary to take care of it.