Taxi to Trinity College (Ireland) from Dublin Airport

Around 25-30 €. Alternatively, you could take an Aircoach bus for 6 €.

If you are interested in the bus option, here is a link with more info.

https://www.aircoach.ie/route-700-dublin-airport-dublin-city-centre

I took the bus a few years ago. I was mid-sixties, in good health and had a carry-on that I could manage by myself. We stayed at Trinity College. The bus stop, as I recall, was just outside the entrance to the College.

Another recommendation for the bus. We got off at St. Stephens Green. Easy and inexpensive.

Regarding the Aircoach bus, I also wanted to mention that when I bought my ticket into Dublin from a kiosk at the airport, the ticket seller asked whether I wanted a return ticket as well. I didn't get one then, but later, when I realized that I needed one, I was able to reserve a ticket online, and catch the bus right in front of the College. They also go to the Dublin airport from other cities.

The Airlink Express also provides €6 one way per adult from Dublin Airport to many stops in Dublin:

http://dodublin.com/transport-dublin-ireland/travel-cards-airport-transfer

We bought the DoDublin Combo Package that included the Airlink Bus, plus the 48 Hour Hop on Hop Off option.

We like DoDublin HoHo as a way to get familiarized with the city on the first day, then decide where we really want to spend our time.

Additionally, the DoDublin HoHo tour includes discounts at popular destinations.
Be sure to read what is and isn't included in the discounts....
Good luck!