Then he must declare is UK income and only declare his Irish income IF he draws a a salutary or dividends or benefits or paid out expenses which you state he does not
Your three questionsI will reply as direct answers to the points as you raised them
1. On his personal tax return will he have to or should he declare the companies he is a director of in Republic of Ireland
NO as there is no income or other payments arising so no need to declare the position itself
2. What are the implications of doing so. Will he have to furnish information about what he has earned from those Irish companies, ie show to HMRC copies of the Irish tax returns and bank statements etc ?
No implications - as there is no income. If you were to advsie its position in the white space of the tax return or HMRC are aware of the position and have queries at which point then, then they have the right to ask for bank statements and Irish company returns
3. Are the earnings which he declares from the Irish companies taxable, even if he has not brought or transferred those earnings into the UK and on what rate of tax is this applied ? So lets assume he has earned 70k after tax from the Irish companies, How would this be taxed in the UK, if at all ? Does it make a difference that he did not bring or transfer any of that money to the UK
Yes absolutely, as he is a UK citizen so liable to tax on worldwide income whether remitted to the UK or not, but this is confusing as you state he has no income salary, dividends or other payment from the Irish company ???? He would pay tax at his highest rate (when combined with other incomes)
So if the combined oncome was in excess of £45000 then 40% isf more than £100,000 then loss of personal allowances (£1 allowance for every £2 over the £100K limit) and 45% tax and loss of personal allowances for income in excess of £150,000.