Particularly CGT on ETFs with deemed disposal etc.?
Firstly, in Ireland CGT and deemed disposal are completely different taxes. So straight off you'd have to build 2 separate solutions for the Irish market, and be able to explain to customers that they've 2 completely different tax filings to complete.
Then you'd need to be able to access all transactions, regardless of the broker the shares are with. There is no public record of shares that can be accessed, or you don't have personal wallets that record all your transactions in one place. And transactions that aren't via brokers; e.g. employee share schemes.
And handle all actions on shares; share splits, DRIIP, rights, etc.
And there's no point doing CGT without covering dividends, and witholding taxes while you're at it.
And then overlay the tax rules for each country, etc.
You'd end up having to charge more than an accountant by the time it was done.
Or, you just use the reports that some brokers already produce if you only use 1 broker; e.g. DeGiro
Or keep track in Excel - it's not that complicated unless you're buying and selling shares on a regular basis. There are Excel templates available from many private sources for both CGT & deemed disposal, but I've no idea which ones actually work.