As I understand it, if you receive dividend income, you are further liable for:
1. the excess of your marginal PAYE rate over the Dividend Withholding Tax already paid (20%)
2. PRSI, and
3. the Income Levy
If you receive dividend income from a UK resident company, a withholding tax is levied at source, but you cannot use this to offset your PAYE liability here. That is, if you are a higher rate tax-payer you must pay 41% of the income. In effect you are taxed twice, once in the UK and once here. I am unsure if a similar situation exists for income earned in the US or elsewhere.
From a personal perspective, I just completed my first declaration of income (Form 12) and was amazed at the taxation burden on dividend income. By my calculations, if you factor taxation (at the higher rate) into the dividend yields of most ISEQ and FTSE companies at the moment, then they do not compare at all favorably with cash on deposit.
In future I would be more inclined to invest in stocks that are not "high yielders" - it seems to me more efficient to seek long term gains via capital gains - ie growth stocks. Of course, there are a million other variables when investing, so this opinion is very much an "all things being equal statement".