Yes, rugby union is played in Ireland
Yes. Ireland is one of the world's leading rugby union nations.
Ireland has many rugby teams. However, what you are probably referring to is why there is not a Republic of Ireland and a Northern Ireland team. The reason is that, like many sports in Ireland, rugby is played on an all-Ireland basis and there one organisation that looks after it, the Irish Rugby Football Union, and so there is one international team.
All-Ireland League - rugby union - was created in 1990.
Its the IRB (International Rugby Board)
If they are both union code ... nothing. Youth rugby is played on the same basis as full international regulations
Fiji is the shortest registered rugby union team. Arguably the USA but that is an abbreviation.
Blue and yellow