depends where you are coming from, the most direct is the tallaght exit at spawell, it is straight out to Blessington from there. Depending on traffic it could be anything from 25 minutes to 1 hour and 25 minutes. The main bottlenecks are from after the Square to Citywest and entering Blessington itself can be somewhat slow.
Alternatively you can take the Red Cow exit and turn left after Citywest business park, past the Citywest hotel, this will bring you along a back road through Saggart village and on to the N81 about one mile the Dublin side of Brittas.