First of all, you never need those brackets after the = unless you're using a function, which you're not.
findclientbyname("Danny Boy").clanprefix="<color:FFFFFF>IRA";
Next, you do not need to put in your full name:
findclientbyname("Danny").clanprefix="<color:FFFFFF>IRA";
Now, for a few questions:
-Are you trying to copy and paste the line of code? You cannot copy and paste <tags> into Blockland. You must retype it.
-Are you the host of the server, putting this into the server's console (if dedicated)? Or are you on someone else's server using eval?