I can confirm it is a perfin used by Arthur Guiness Son & Co Ltd, Dublin, and by its branches in the United Kingdom.
The entry comes from the Perfin Society Publication Perfins of Ireland, currently out of print but a new second edition E version will be available from mid March 2022 to both perfin society members and the public at low cost.
GB Perfins arenot given a price or rarity rating, with nearly 25K different perfins it would be an impossible task. Rough ideas on price can be got from viewing past Perin Society Auctions with prices realised on their website. As there are no other Seahorse perfins reported with the Irish Free State overprint I expect it would fetch a good price in the Perfin Society Auction, but still a lot less than if it didn't have a perfin and sold as a normal stamp.