That's not entirely true.
First of all, when Apple pays income tax in most countries, they will receive tax offset credit in USA so they don't have to pay tax twice on same profit.
But Apple doesn't even do this. The whole point of having Irish subsidiary is to give it intellectual property which is the real money maker here.
What happens then is quite simple. Subsidiary company in Ireland is charging Apple in other countries huge licensing fees. In fact, so huge that it amounts to 100 billion dollars. Licensing fees are tax deductible so Apple is basically avoiding paying corporate tax all over the world because licensing fees and royalties eat up all the profit.
Of course, Apple as a group of entities is making billions in profit but as far as overseas tax authorities (Australian Tax Office, UK Revenue & Customs etc) are concerned, Apple is barely breaking even therefore is not paying its fair share of tax.
Obviously this whole scheme comes at cost. All these billions are basically stuck somewhere in Ireland and can't be used. They just sit there and earn interest.
Eventually money will have to come to America. How? Irish subsidiary will pay dividends to parent company Apple Inc. At that point all these billions in profits will get finally taxed at ~35% by IRS.
Congress is just asking Apple when they are going to bring to USA all those profits and what's the point of deferring it. Apple is saying, we will bring it when you lower our tax rate. They are basically blackmailing the government.
edit: I was wrong by saying Apple doesn't pay corporate tax in USA. I modified my comment to reflect what I have learned from comments bellow.