lyonsa3, I presume you mean that there's a rent ceiling of €150 per week in your area for your type of accommodation & the family size of your tenant (single, couple, etc).
If you declare your rent charged below the going rate for the area, the Community Welfare Officer will know it's not correct because they will be aware of the rents in their area, and will most likely query it further (it may be seen as collusion between landlord and tenant to receive rent supplement when the tenant is not entitled to it).
There are different rent limits for different family sizes, what applies to a single person would not apply to a couple, family with children etc. The limits also vary by county.
The link to the current maximum rent limits may be found here
www.welfare.ie/topics/legis/si44_07.pdf
(PDF document, requires Adobe Reader). See page 4 of the doc for the limits.
The Rent Allowance link is to a different scheme - the payment the OP refers to is Rent Supplement, paid under Supplementary Welfare Allowance. The confusion arises because most people colloqually call Rent Supplement "Rent Allowance".