Day return is 4.30, so its cheaper in cash terms to buy a 7 day ticket at 20.60 than a monthly
The exact savings depend on what tax rate you pay, you save on PRSI and income tax.
Take your monthly pay before tax, subtract from that the value of the ticket, the number you get is the amount your tax is now calculated at. The income tax and PRSI you pay are both reduced as a result. There are some examples
http://www.taxsaver.ie/ie/how_much_can_i_save.asp. Basics are you will save around 24% rising to over 45% if you pay at the higher tax rate
Note the monthly ticket would be valid on all trains in the semi circle formed by Maynooth, Balbriggan, Kilcoole and Hazelhatch, so if you head into Dublin on the weekend for instance its effectively free
The annual ticket is typically equally to 10 months in cost sometimes less, 890 in this case