The regular, full-time IRS employees will be at work Monday no matter what happens on the budget. That is promise. Want to put your money where you mouth is?
https://www.google.com/amp/s/amp.cnn.com/money/2018/01/20/pf/taxes/irs-taxes-government-shutdown/index.html
Looks like 43% of the IRS are working because it is tax season. The rest are off.