Ard-Ri is a diplomacy variant set in Ireland which starts in Winter 379 A.D (the winter is an adjustment phase, and therefore players can choose their own units to be placed in their supply centres). The 5 Irish powers are Ulaid, Connacht, Midhe, Laigin and Muma. If a 6th player plays then another power, the Vikings, are added. There are only 15 on-board supply centres, and holding all 15 is the requirement for victory! (Victory can also be ceded to one player by agreement of all others, but when would that ever happen ;-)
The game is fast and bloody. There are no neutrals who can be attacked first (for easy centres) and thus relations with your neighbours are the only priority. You don't have a turn's grace (like in Standard Dip) where you can say, "I'll just pick up my neutrals and wait to see what happens".
Apart from the no-neutrals difference, there are also the extra raiding rules. They allow players with fleets bordering the edge of the board (in an ocean) to order their units to be 'raiding' instead of the normal diplomacy orders (of Move, Support, Hold, Convoy which are all available too). A raid can only be performed in the Spring and this MUST be followed in the Fall by a 'raiding return' order for the same fleet (which can be to any of the 4 outermost seas, regardless of where the raider left the board). If the raider is bounced (by a unit in the sea they return to or a unit ordered to the same sea, even by a raid return) then all raiders so bounced are considered to have no retreat and hence are eliminated. If a raider does return successfully then the player is awarded a temporary extra supply centre for that year (for each fleet that raided and returned; thus there are at most 4 extra supply centres available each year).
When a 6th player is required the Vikings become a power. The vikings start with 3 "off-board" supply centres, and these are reduced every time the viking gains one of the on-board SCs (thus necessitating the viking be assured of holding any mainland gains, as the viking player doesn't get the off-board centres back if they lose an on-board one). While the vikings might sound like a great power they have a number of restrictions which make them difficult to play. They must write their builds and reveal them (which have to be fleets in one of the 4 outermost seas), but they are unable to be placed until after the full Spring moves, including retreats, are performed. The difficulty being that any unit existing in the target sea causes the build to fail! Once the viking gets on the board (difficult in itself), then they face the added challenge of being unable to go raiding (thus limiting their supply centres to only what they hold). The viking player may in the Winter builds phase change any fleet, located in a land area, into an army (again this is dangerous as an attacked army will likely have no retreat from such a coastal province). The viking is a hard power to play, but this is mitigated somewhat by the fact that they can attack the weakest areas and are allowed to build in any supply centre they hold.
Ard-Ri is Diplomacy with no lines and lots of outflanking (by both the Vikings and the returning raiders). Your back is never safe and there is never a stalemate line to hold. As such strong relationships with the other players are hard to form, but vital to success. There are only 13 landlocked provinces, out of 33 land provinces; this makes naval power a hugely important part of the game. Naval power is even more important when considering that a fleet can be on the other side of the board in one turn (due to raiding). However, there are few sea areas and having too many fleets means that you are weakened on land, and this is dangerous, considering that is where the 15 supply centres (for victory purposes) are. A fine balance of land and sea power must therefore be maintained.
All in all, quite different from diplomacy, slightly difficult for the 6th (viking) player, but good if you're looking for a change (and most suitable if you only have 5 players).