W8-BEN gets held by your US broker - it simply authorizes them to not apply US withholding tax to any gain you make. I would be highly surprised if the form every makes it way to Revenue - it's a form that the Broker must be able to show to US tax authorities (IRS) - but as there is a tax treaty between US/IRL, you never know.
No PAYE tax is deducted.
There are generally 2 such stock schemes: RSUs (Restricted stock units), and Stock options.
Stock options - your company gives you the right to buy shares in it's stock at a predetermined price, say $X. If you exercise them at a price greater than $X (you'd be nuts to exericise below $X) you then are liable for benefit in kind tax on the gain - and have to fill out form RTS01 (google search that and you'll find it) & must submitt payment to Revenue within 30 days.
You are also obligated to file self-assessed taxes (i.e. form 11) for that tax year.
RSUs... you get the granted free shares in ur company... and get to sell them whenever the right vests. This is similar to a stock option with strike price of 0, however you do not have to pay tax within the 30 days - but have to pay by Oct 31 of the following year (but you may need to pay preliminary taxes).