Hello Andrew,

Let me start with the Northern Ireland problem - I looked at the secondary data source and I do not see any postcodes for Northern Ireland in that data set.   As for how to do filled shapes in Tableau for UK data sources, this is not directly supported in Tableau but you could use the techniques described here:   Re: Filled maps per postcode in UK   -

The data blending  solution may cause issues for you such that you may consider creating a Join between the two sources of data. 

I hope that helps.

Patrick