Almost certainly, yes. ESPP shares are normally acquired at a discount (usually of at least 15% but maybe more depending on share price movement during the purchase period) to the market value on the date of acquisition and that difference is assessable for income tax/PRSI/USC. As mentioned above in some cases your employer will deal with this via payroll. Similarly if by vesting you mean that you are taking ownership of shares and these are free or at a discounted exercise price compared to the market value on the date of acquisition then, again, this difference is treated as income and assessed as such.