Ham Slicer is indeed correct. The assessment is based on the cash amount received exclusive of any tax credit in respect of dividends received post 5 April 1999.
Under the terms of the Ireland/United Kingdom Double Taxation Agreement, the dividend may be taxed in the UK at a rate not exceeding 15% of the gross dividend (i.e. inclusive of the tax credit), however under UK domestic law, this liability will be reduced to an amount equal to the tax credit which is attached to such dividends. This credit currently stands at 1/9th. The Irish tax resident recipient of the dividend will in effect suffer UK income tax equal to the tax credit attached to the dividend post 5 April 1999, with no entitlement to a repayment of the tax credit. The net dividend received will be subject to Irish income tax in full.
For example:
Gross Dividend received €100
Less UK Tax of -10
Net Dividend received 90
Irish tax @42% -37.8
Actual dividend 52.2
That's where OnDeBanks gets his figure of 47.8%... 10+37.8