I think this is how it works:
Put a market value (MV) on the items that you will likely realise on disposal.
Compare this to the current tax-written down value (TWDV) i.e. Expenditure minus (no. of years capital allowances claimed x item cost x 12.50%).
So, if you acquired a washing machine for €600 and have claimed two years of capital allowances (2 x €600 x 12.50% = €150), the TWDV = €450.
If the MV you realise on disposal is €500, then you have claimed too much in the way of capital allowances against your income and need to account for a balancing charge in your Case V rental income calculation this year. So, add €50 to your rental income for 2021. (EDIT: no balancing charge arises where individual fixtures and fittings is disposed of for €2,000 or less to an unconnected person.)
If the MV on disposal is €250, you have not claimed enough allowances and so a balancing allowance of €200 is available as a deduction against your Case V income for this year.
If your tenant has vacated and you have no income to deduct this balancing allowance from, I believe you can take a deduction in your CGT calculation on the sale of the property for the full cost of the item (€600) LESS a deduction for the capital allowances claimed to date (€150) and the balancing allowance on disposal of €200. So, you can include a deduction for €250 net in your CGT calculation which will reduce the CGT payable or increase your loss (if that is the case) for carry forward to offset any capital gains you may have in the future.