There are a few options from McDowall and it would depend on what part of McDowall you live in. You have the option of either catching a bus (which would cost less than $6 each way) or driving as there are no train lines in the area. Your closest train station would be on the Ferny Grove line.
You can find the bus routes and times on www.translink.com.au or if you wish to drive, I think the below options are your best ones;
If you get on Old Northern Road you can follow it through Everton Park, Enoggera, Ashgrove, Red Hill and Spring Hill before you hit the city.
You could also go out on Rode Road, head south on Appleby Road, follow Shand Street / Enoggera Road through to the city.
Otherwise Hamilton Road goes straight through to Gympie Road, once on Gympie Road you head south into the city.