If an Irish based invested converts a sum of money in their trading account into dollars how does this affect the transactions of dollar denominated stocks - with respect to the fluctuations in the Euro/Dollar rate over time? Here's an example, where I've included my guess at how it should treated in italics within square brackets...
To me your calculations look broadly right with one exception. I am going on my reading of Revenue guidelines, not on any qualification or expertise in this area. The mains ones would be [broken link removed] and [broken link removed] (note: links to Revenue website searches as Revenue documents have a habit of moving).
The first point is that CGT liability on foreign currency share trades is based on exchange rate on date of acquisition and disposal of the shares. So your italicised guesses on examples 2 and 3 conform to this, i.e CGT on shares arises when you sell the shares and is based on euro-equivalent price on dates of acquisition and sale (
of the shares, not the dollars).
Regarding currency trades, the second document applies: "
a chargeable gain/allowable loss can arise to a person buying and selling foreign currency otherwise than in the course of trade. That gain/loss is computed by reference to the corresponding euro value of the purchase price and the sale proceeds". Your italics on 1,4, and 5 look to be in accordance with this, i.e. capital gain or loss occurs when you convert back to euros, based on difference from original purchase price of dollars.
The one that looks wrong (though I am unsure) is your example 6. The 500 USD was effectively bought with euros on the date of the sale of the shares. After all, that's the amount you declared tax on. So arguably a capital gain or loss occurs when you convert the 500 USD to euros, based on the difference from the exchange rate from date of sale of shares. On the other hand, that Revenue note specifically refers to "
buying and selling foreign currency otherwise than in the course of trade". If the 500 USD is considered to arise in the course of trade, then it does not apply. I've no idea if this is the case or, if it isn't, what other rules apply. My gut feel is that "trade" refers to the trade of goods and services and not shares, so CGT does apply as I suggested.
If it was me, and I was looking at, say, a ten cent movement in the exchange rate, so the amount of tax involved was very roughly on the order of 500 * 0.1 * exch_rate * 33% =~ €15, I'd completely ignore it on the basis that life is too short to be
that scrupulous. If it was one or two orders of magnitude more, I'd ask Revenue or pay for advice.