we are arriving about 8am on a Tuesday the middle of September and wonder about the time needed to clear customs, immigration, and board the Thalys to Brussels.
Assuming that your flight is on time and assuming that your baggage is not in Dublin by mistake allow anywhere between 40 to 90 minutes to clear passport and customs. The train station is located in the airport. You may need to take a short one way train ride to Amsterdam Centraal Station to connect with your train. It takes about 20 minutes from Schipol to Amsterdam Centraal Station.
I've never had to wait more than 30 minutes to clear immigration and collect baggage at Schipol. The train station is literally right outside the immigration hall. All Brussels bound trains stop at Schipol so it shouldn't be necessary to take a train into Amsterdam Centraal. I would take the regular inter-city train to get to Brussels. The Thalys is usually twice the fare, only a little bit faster, and and doesn't run as frequently. The IC trains run every half-hour; the morning and evening rush hour, they run every fifteen minutes. Advance reservations aren't necessary simply purchase your ticket at the train station.