the main catholic church in doha is located in abu hamour area, the easiest way to get there is thru the E-ring road, if you're coming from the airport just turn left on woqood r/a and you won't miss this big church. but since there are ongoing construction work in this road, your best alternative will be to go straight on woqood r/a, make a u-turn on the next r/a then turn right to the last exit before reaching woqood again. there you'll find an obvious access towards the church. be careful though, bcoz it is just plain rocks & sand, just follow the trail, anyway the church is quite big to miss. cheers!