Eligibility for inclusion in the supplement to the register is determined by eligibility on the date of the application to the registration authority. For practical and administrative reasons there has to be a cut-off point in advance of polling day for the receipt of applications for inclusion in the supplement to the register so that they can be processed by registration authorities.
The Electoral (Amendment) Act 2015 addressed an anomaly that previously prevented naturalised citizens from registering to vote in their capacity as Irish citizens where they had not become citizens by the qualifying date for the register (1 September annually). The 2015 Act enables such naturalised citizens to apply for inclusion in the supplement to the register but they must do so before the closing date that applies generally for inclusion in the supplement.
Mindful of the practical considerations and the need for a cut-off point prior to polling day, I currently have no plans to change electoral law to enable the registration of persons in circumstances where they are naturalised after the deadline for inclusion in the supplement to the register of electors but before polling day.