How can Pharisees turn someone from God if His grace is irresistible? How does Calvinism reconcile these words from Jesus along with its doctrine of irresistible grace (IG)?


  Matthew 23:13,15 ESV But woe to you, scribes and Pharisees, hypocrites! For you shut the kingdom of heaven in people's faces. For you neither enter yourselves nor allow those who would enter to go in. Woe to you, scribes and Pharisees, hypocrites! For you travel across sea and land to make a single proselyte, and when he becomes a proselyte, you make him twice as much a child of hell as yourselves.


I bolded the parts that seem to say that it is the Pharisees who are causing these people to turn away.

The reasons why I think this passage and IG are at odds is because if the person being proselytized...


...did not already receive God's IG, and was therefore not elect to begin with, why would Jesus say that the Pharisees make them a child of hell?
...did already receive God's IG, how could any actions from the Pharisee's interfere with that?
The concept of Irresistible grace is summed up this way:


  Although the general outward call of the gospel can be, and often is, rejected, the special inward call of the Spirit never fails to result in the conversion of those to whom it is made. This special call is not made to all sinners but is issued to the elect only! The Spirit is in no way dependent upon their help or cooperation for success in His work of bringing them to Christ. It is for this reason that Calvinists speak of the Spirit's call and of God's grace in saving sinners as being ‘efficacious,' ‘invincible,' or ‘irresistible.' For the grace which the Holy Spirit extends to the elect cannot be thwarted or refused, it never fails to bring them to true faith in Christ! (Steele and Thomas, The Five Points of Calvinism, Defined, Defended, Documented, p 48-49)


The important thing in the passage you quote is that the people are responding not to an inward call, but to an outward one. If they become "twice the child of hell" there could not have been an inward call in the first place.

To answer your addendum, there's no suggestion that the person isn't already a child of hell, they are made worse by the Pharisees. That's the point here. The point of this passage isn't to make a doctrinal statement (so it would be kind of dangerous to infer doctrine from it), but it's to point out the pharisees that their attempts at proselytization are so horrible and backwards they end up pushing their converts further into sin and condemnation than they already were!