It’s not a good idea to ignore the tax. The IRS will be able to tell from your more recent tax returns whether you should have included the Repatriation Tax on your 2017 returns. And the IRS has already issued public warnings to say that it will pursue taxpayers who did not report and pay the Repatriation Tax in 2017. You can generally obtain a much better result if you fix the situation voluntarily before the IRS audits you.