There were actually two Clubhouses in the old Lansdowne Road. The one in that photo is Wanderers FC ( three of whose members won VCs in the Great War). The other one was at the other end of that stand and was Lansdowne FC. (Both clubs are Rugby Clubs, founded before the codes split, so they don't use the "R".
By the way,the top deck of that stand was above the Mainline railway.