I think the rulebook is a great (though expensive for slim volume) d20 ruleset for Celtic roleplay, but somewhat lacking in detail on Slaine's world - to get that you have to cough up more cash for the sourcebooks. In this respect it's a bit like Traveller20 is for semi-hard sf, except Traveller20 is 442 pages plus adds and Slaine is 192 pages, much of it colour plates, at a similar price.