Your brother will only be entitled to arrears from the date he signed on. It's usual to pay the first payment then sort out the arrears. If he claimed SWA while waiting on jobseeker's, anything paid by the CWO will be taken from the JA arrears.
I presume your brother is under 20, as €100 is his full weekly Jobseeker's Allowance entitlement.
He will only be entitled to Rent Supplement if he's been renting privately for 6 of the previous 12 months or is on the Co Council housing list. There is also a minimum contribution (currently €24 pw) which he will have to pay himself if he does qualify. Your brother can make an application for Rent Supplement by contacting the CWO at your local health centre.