A spell that can’t be countered is a legal target for Ionize. The spell won’t be countered when Ionize resolves, but Ionize still deals 2 damage to that spell’s controller.

Ionize : règles, fonctionnement et interactions (MTG)

Ionize is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Counter target spell. Ionize deals 2 damage to that spell's controller.

How it works : Ionize

Original English text — Wizards of the Coast / Scryfall

#1October 5, 2018· Wizards of the Coast
A spell that can’t be countered is a legal target for Ionize. The spell won’t be countered when Ionize resolves, but Ionize still deals 2 damage to that spell’s controller.
No other card is named in this ruling.