Welcome to the forum! Two ways, there is a pass that you will discover once you find the location to quintarra. The other way, well you will have to ask someone else, I never found it... :sad:
yo just follow the story line. Before you can cross those mountains heres what you should have done (going backwards in order): visited stillwater and convinced Myrhh the elf to tell you where Qintarra is, Been to the clan of the wheel cave, been to the isle of despair (where you find out where the clan of the wheel is, and how to get inside), i cant remember exactly what preceeded this...but the point is, just follow all the major quests in order and you will eventually get there. Make sure you talk to The rich guy in the mansion in Tarant, Bates i think is his name, all of the quests he gives you lead up to getting past those mountains. If you complete each quest he gives you, in order, that will lead you up to getting to Qintarra, and the other side of those mountains. I highly recommend you don't try to go to any town or location out of order, because there are triggers, and if you go to some town before the trigger has been set off, important quest NPC's will be missing or will not respond, and you will basically screw up your game, I know this from experience. In other words, never goto a town or special location until an NPC has marked it on your map, even if you know the coords. I wasted an entire day bashing my head against a wall trying to figure out a quest, when I realized I had not set off a critical trigger, and i had to go back and figure out where that trigger was in order to progress any further.
*Spoiler* What you need to do, all from the beginning: Get the ring. Go to Tarant. Go down the basement in the house. Talk to the necromancers. Go to and talk to Gilbert Bates. Go to BMC. Go to Gilbert. Go to Ashbury, I think. Talk to ship captain. Go to Isle of Despair. Win a fight in the pit. Talk to the dwarf. Steal either his glasses or the key to the chest and then take the schematics of the glasses. Go to Gilbert. Go to wheel clan. Go to stillwater. Get a hide of the monster. Go to stillwater. And voila, the passage through the montains is there.
If you happen to be a mage you can teleport yourself to the otherside if you have a location there... but I suggest you stick to the mainplot of the game since a mate tried that and he screw up the game because he didn't do it right!