The Curragh is 3km from Kildare train station, half an hour walk on a narrow road, but it may be possible to walk beside the road, been years since I was down there. I'm sure that the taxis in Kildare will be quite busy. Still, it's do-able, so you aren't dependent on the shuttle bus. Trains are approx twice an hour on sat, once an hour on sun.
Edit: Day return Heuston to Kildare is €19.50 so it's pricy.
There are buses from Bus Eireann which will drop you to the town centre, again about a 3km walk to the Curragh. One bus an hour I believe. Also for those on a budget there is the Dublin Coach from the airport which will drop you to the Outlet centre where you can use the free shuttle bus to bring you to the train station and walk the rest of the way.
Edit: Bus Eireann is €15.20rtn, Dublin coach is €5 each way.
For good beer we will find a way!