from the Prems website:
HOW MANY FOREIGN OR OVERSEAS PLAYERS CAN PLAY IN THE PREMIERSHIP?
It is important to note that the legal definition of foreign players is different to one of non-English players. In addition, players can fall under the Bosman or Kolpak rule. The Bosman ruling reinforced the Treaty of Rome in a sporting context, allowing the free movement of EU national workers around other EU countries. This effectively means any EU national has the same rights as any ‘local’ worker and therefore cannot be defined as a foreigner.
In the Premiership, the rule on foreign players has been amended to come into line with the regulations in European competitions.
A maximum of two foreign players can now be on the pitch at any one time, at all periods during the season.
So Kolpak players constitute players from Fiji, Samoa, Tonga or South Africa.
So, if a player is qualified to play for a country other than an EU or Kolpak country & does not hold a UK or EU passport or is not married to a UK or EU citizen then they are classed as foreign. Unless they are Irish and they are not classed as foreigners under the Common Travel Agreement.
A club can have 2 foreigners in your side
Thats the legal side. In order to qualify for RFU cash clubs need to have a percentage of the team EQP qualified, can't remember the details.
How much this will change come Brexit, who knows. I would assume nothing will change immediately.
Hope thats cleared that up