Got 2 wages in one month, but less take-home

So, I quit my job at the end of November 2015, and had a 3-month notice period. Was put on gardening leave, and agreed that my December 2015 salary would be paid normally, and that the January and February 2016 salaries would be paid together in January. (For a little bit of background - I'm a non-EU citizen and my residence permit was expiring mid-January.)

Thing is, the take-home amount on my January payslip is a good 10% less than if you took my regular take-home amount and multiplied by two. The company's accountant says that this is due to the fact that only January's tax credits were applied, even though 2 salaries were paid out, and that I can reclaim the tax credits if I don't have any income in February 2016. (My official employment ending date is February 24th.)

Can anyone tell me how I can go about getting the difference in take-home refunded from Revenue? From the website, I can see that it would go something like:

1) Wait 4 weeks after my employment ends;
2) Send them a P50 and my P45;
3) The refund the money into my Irish bank account.
(I'm pretty sure I can't do this online through PAYE anytime.)

Can anyone enlighten me a little bit more? Thanks!