4chan

There are some subtleties to this particular topic that are worth mentioning. I would be careful to distinguish between constructing vs defining here.

The usual definition of the irrationals works roughly like this:

You have a set of numbers R which you call the real numbers. You have a subset of the real numbers Q which you call the rational numbers. You define a real number to be irrational if it is not a rational number.

This is perfectly rigorous, but it relies on knowing what you mean by R and Q.

Both R and Q can be defined "without" (a full) construction by letting R be any complete ordered field. Such a field has a multiplicative identity 1 by definition. So, take 0 along with all sums of the form 1, 1+1, 1+1+1 and so on. We can call this set N. We can take Z to be the set of all elements of N and all additive inverses of elements of N. Finally take Q to be the set containing all elements of Z and all multiplicative inverses of (nonzero) elements of Z. Now we have our R and Q. Also, each step of the above follows from our field axioms. Defining irrationals is straightforward from this.

So, the definition bit here is not a problem. The bigger issue is that this definition doesn't tell us that a complete ordered field exists. We can define things that don't exist, like purple flying pigs and so on.

What the dedekind cut construction shows is that using only the axioms of zfc we can construct at least one complete ordered field.