Hap rates vary by local authority, google HAP rates Carlow, or wherever.
Wether he will be seen as a single person or as having 3 dependents will of course determine the rate HAP will be approved at. There is usually a special rate in excess of the published rate, but there would have to be a particular reason for the to be authorised.
By the way as a landlord of many years experience I strongly advise against this. A friend, a hard luck situation, disaster. If things go south for your friend protecting your interests will, understandably be the least of his concerns.