The Employer P35 deadline is 15 February. However, as advised in eBrief No. 10/2009, the deadline for the P35 has been extended to the 23rd of the month where the return and associated tax payment is made via ROS; whether voluntarily or under the new mandatory regime.
Of course, it is important to note that where the P35 and associated payment are not made electronically by the new extended deadline, the extended time limits will be disregarded so that any interest imposed for late payment will run from 15 February and not 23 February.
If you need assistance call the Revenue P35 Helpline on 1890 25 45 65. Further information is available here