Hi Charlie Hey,
Self employment is covered under article 20 - 'income not expressly mentioned'.
The subject to tax rules do not apply in relation to Ireland.
You enter the amount of foreign income that the certificate is for (future dates cannot be used).
In your tax return, this income is declared in the self employment section.
You would only need to include in the foreign section as well, if you were required to pay foreign tax, as a result of the tax treaty.
Please also have a look at the guidance at
DT9850PP - Double Taxation Relief Manual: Guidance by country: Ireland for more information.
Thank you.