Trigger to update only if value is in lookup table
You could change the trigger to only update when the NameOfParameter matches the Tag from TagDescLookup 

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TRIGGER [dbo].[TagDescTrigger]
    ON [dbo].[EventLog]
    AFTER INSERT
    AS
       BEGIN
       SET NOCOUNT ON

        UPDATE E
        SET E.Comment =TDL.[Description]
        FROM
        [dbo].[EventLog] AS E
        JOIN INSERTED AS I
        ON E.NameOfParameter = I.NameOfParameter
        JOIN TagDescLookup AS TDL
        ON TDL.Tag = I.NameOfParameter;
        END
    END


You really need to look out for duplicate Tag's.



Testing

CREATE TABLE  TagDescLookup 
(ID int NOT NULL PRIMARY KEY IDENTITY (1,1)  ,
 Tag nvarchar(MAX) NOT NULL  ,
 Description nvarchar(MAX)  );

 CREATE TABLE [dbo].[EventLog]
(Comment  nvarchar(MAX),
NameOfParameter nvarchar(max));


Example1

  INSERT INTO [dbo].[EventLog](Comment,NameOfParameter)
  VALUES('bla','bla');

  select * from dbo.EventLog;


Result1

Comment NameOfParameter
bla bla


Example2

  Insert into TagDescLookup(Tag,Description) 
  VALUES('TaggedVal','NewDescription');


  INSERT INTO [dbo].[EventLog](Comment,NameOfParameter)
  VALUES('OldDescription?','TaggedVal');


  SELECT * From dbo.EventLog;


Result2

Comment NameOfParameter
bla bla
NewDescription  TaggedVal