Standard US ESPP's usually have an option element - that's why Revenue group them under RTSO. If you've something called an ESPP without an option element - then I'd proably expect Revenue to still lump it into RTSO.
An ESPP typically would allow an employee to buy shares with their own money at the cheaper of the start price or 6 month end price. That's a type of share option. This option aspect is why I sign up for every ESPP, I'd do it even if the discount was 0%.
However if all you have is a 10% discount - with no option to buy at a cheaper price, the scheme isn't as attractive as it could be especially for Irish workers. Here the discount is taxed at ~52% and payable after 30 days, and you need to do an RTSO and Form 11 - you pay any fees involved in selling the shares yourself and Revenue ignore those costs.
If you were just buying 300 dollars a 10% discount wouldn't be worth it, even 1000 dollars with 100 dollars in profit - 48 dollars after tax and before fees?
Next year Revenue are going to insist all ESPP have the tax paid by the employer btw.