Past Debt/ New marriage

Generally you are not responsible for the old IRS debts. However, if you comingle your assets, you can end up getting stuck for some of these debts. For example, if you purchase a home together or have a joint bank account, the creditor can lay claim to these assets. Until the debts are worked out, you should keep your assets separate.

The prior answer should be taken not only as a suggestion, but a requirement that should be done immediately.

Robert Scott Williams answer is right on target. Also, for future reference, if you are getting married and are worried about your spouses past debts you can execute a prenuptial agreement. A lot of people shy away from prenups but they can be very helpful, and it can be drafted to your specifications. In your case, (still follow advice re: commingling assets), you could have drafted an agreement that basically said "her debt is hers" and "my debt is mine". This will not help you, but you may be able to help out a friend who is getting married.