WordWeb Online

1. Causing irritation or annoyance
   - annoying, bothersome, galling, irritating, nettlesome, pesky [informal], pestering, pestiferous, plaguy, plaguey, teasing, vexatious, vexing, niggly, dratted [informal]
    
2. So lacking in interest as to cause mental weariness
   "what an irksome task the writing of long letters is";
   - boring, deadening, dull, ho-hum [informal], slow, tedious, tiresome, wearisome, unamusing, deadly [informal], draggy [informal], mind-numbing

See also: disagreeable, lame, lameo [informal], meh [informal], samey [Brit, informal], uninspiring, uninteresting