They don’t want to categorize it as social security/pensions/health related or otherwise the hole in the PRSI fund disappears, and they’d have surplus cash to fund the health service. The USC is being used for general spending not for one particular problem.
But they also don’t want to categorize as income tax as they don’t want it affected be income tax banding or reliefs.
The closest international comparison I’ve seen is the French very similar 7.5% CSG, contribution sociale généralisée . This may be where the Francophile Brian Lenihan got the idea.
However the French do use that tax for their social security. When you hear about French people paying large amounts in social security they include their CSG in that figure.
Possibly when we eventually do balance the budgets the USC will switch to funding social security.