In relation to Rent Supplement, he cannot claim a supplement if he's living at home.
If the maximum rent limit for Dublin is €98 per week, it is unlikely he will receive Rent Supplement for a property of €700 per month on an ongoing basis. He may get a few weeks' grace because he's just become unemployed to allow him to re-negotiate the rent or find somewhere cheaper.
Rent Supplement is not payable if the rent is in excess of the limit set for the county. If the landlord charges a different amount to the amount he declares on the Rent Supplement Application form (the landlord must complete one section of the form) it could be seen as facilitating Social Welfare fraud. It would also place a very heavy burden on your nephew, requiring him to pay a very large amount to rent from his SW income.
He should make contact with the SW Local office as soon as he becomes unemployed - I understand the offices are operating an appointment system, and will advise him when to come in. You'll find very useful information on signing on in the sticky post at the top of this forum.