Probably best to buy a [broken link removed] ticket (€6 from the ticket machine at the airport*), get any of [broken link removed] and then get a [broken link removed] from the city centre (or maybe get the 746 towards Dun Laoghaire if there are feeder buses to Tallaght from out southside?). If you don't get the 746/7/8 then you can probably just pay the normal fares and spend less than €6 in total.
Note that if you get the 746/7/8 and pay on the bus then I think your €6 goes on that journey alone and you don't get a Rambler ticket allowing you to take further bus journeys on the same day.
If you are a sporadic bus user but take a few buses on the days that you do use them then the 5 x one day Rambler pack for €18.30 is good value compared to the normal price of €6 for each individual Rambler.