Sorry but it is not the "same offence". It is the same nature of offence but it was for many less persons and it stopped relatively soon as some French contracts were given (as in easyJet also condemned for same reasons) whilst Ryanair continued not to meet the European legislation.
EU Rules only apply to any people appointed after the date the legislation becomes effective.
"The
European Parliament passed legislation governing the payment of employee and employer social insurance costs in May 2012. The legislation governs the country in which employees and employers must pay social insurance costs. Before Jun-2012, Ryanair always paid social charges in either the UK or Ireland, the two countries under whose laws it draws up its labour contracts. Under the terms of legislation introduced in Jun-2012, employees and employers must pay social insurance in the country where the employee is based, although those employed prior to the introduction of the new legislation are exempt."