Get the number 8 bus to Mayfield. Walk down the hill away from the main supermarket for 10 minutes. You will arrive at two petrol stations (Topaz and ?). You can ask around there for people headed east. If they're going east, but in the wrong direction after the Dublin and the Waterford road are separating, have yourself dropped of at the second roundabout (works great for Dublin, don't know about Waterford).
Go to the Blackpool shopping centre on the North side of the city. This can be reached by the number 3 bus from Cork City Centre (Merchants Quay) for around 1.50 Euro. From here, you walk up the motorway for 10 minutes until you reach a bus shelter on the left hand side with the massive area. Also, try the traffic lights all along this road. We waited for 20 minutes with a sign at this spot and got a lift to Limerick city
Go to traffic lights after Bandon roundabout. Served by bus 8 and 19 to Bishopstown. Leads to N22 motorway.
Southwest towards Bandon, Clonakilty
Go to the Bandon roundabout. Served by bus 8 and 19 to Bishopstown. Leads to N71 motorway.