The collection powers of the IRS are nearly unlimited, which according to my sources means that any money in your Roth IRA or other retirement accounts is fair game if you owe unpaid taxes to Uncle Sam. However, before the agency goes after those funds, it's likely to try to collect what you owe by garnishing your wages, seizing any tax refunds you may be entitled to, putting a lien on your real estate, and so on. Meanwhile, as long as you have an outstanding debt with the IRS, interest and penalties on that debt will accrue significantly increasing what you owe.
The best way to deal with an IRS debt is to contact the agency to try to work out an installment payment plan. If you don't feel confident doing your own negotiating, get help from a tax CPA or a tax attorney.