From Tirana to Kruje and back you can get by bus. Near Tirana's train station there is a dusty square with many buses - so this is NOT the depatrure place for buses to Kruje. Near this square there is a bazar entitled Market Shkoder. You need to go through the whole market until you see another dusty square with buses and cars parked. At this square drivers do not shout their destinations, in fact there were no people at all when we came there (12:30 pm). You just need to find a bus with "Kruje" written on it and wait. There is a defined schedule as far as I could understand. Our bus left at 13:00 and it seems that buses go to Kruje once an hour. The trip lasted about 45 minutes and costed only 100 leke.
We were told there are buses back to Tirana at 15:00 and 16:00. But I don't know if this information is reliable since we took a taxi to get back.
Kruje is about 45 minutes or 1 hour away from Tirana and it's very easy to reach by minibus (furgon). The cost is about 150 lek per person (basically peanuts) and furgons leave as sson as they are full.
Ask a taxi driver to take you where the furgons leave from and don't worry if no one else comes along - even if you have to go by yourself and pay for the total number of seta it won't cost you more than 10 euros.