As for emigration which is something else than just working in another country for a couple of years, the CAIN-website has figures for the '90s.

Roughly 13,000 leave Northern Ireland permanently each year, but more than 90% of them for some other place in the UK (why the CAIN-website calls that 'emigration' I don't know). The remaining less 10% are not specifies any better than 'outside of the UK. But even assuming unrealistically that all of them leave for the Republic of Ireland that is not a big figure.

Wolfgang