If you are over the threshold for registration €37,500 for services and €75,000 for sale of goods (since 01/05/08), then you should have been registered and back registration is possible.
If you are under the threshold for registration then registration is voluntary, not mandatory, and registration can only be effected from the next available VAT period.
Be aware, that where a service involves inputs at the standard rate and outputs at the lower rate, the 2/3rds rule needs to be considered. ( see Revenues Guide to VAT ) www.revenue.ie/en/tax/vat/leaflets/vatguide_2008.pdf