Help towards Rent Deposits may be considered, there is no entitlement to same, each case is dealt with on its own merits.
Asking the landlord to underdeclare rent may be seen as facilitating Social Welfare fraud, as the regulations state that no rent supplement is payable if the rent exceeds the limits set down for the family size & county.
In general persons in employment are not entitled to Rent Supplement - there are some exceptions though (certain training/employment schemes, persons waiting on RAS accommodation), so check with the CWO.