I have written some papers on the subject, see the following. You may download their PDF files from my home page 

sci.ui.ac.ir/~a.abdollahi

Alireza Abdollahi, M.J. Ataei, S.M. Jafarian Amiri and A. Mohammadi Hassanabadi, Groups with a maximal irredundant 6-cover, Communications in Algebra, 33, No. 9 (2005) 3225-3238. 


Alireza Abdollahi and S.M. Jafarian Amiri, On groups with an irredundant 7-cover, Journal of Pure and Applied Algebra, 209 (2007) 291-300. 

45. Alireza Abdollahi, M.J. Ataei and A. Mohammadi Hassanabadi, Minimal blocking sets in PG(n,2) and covering groups by subgroups,  Communications in Algebra, 36 No. 2 (2008) 365-380.  [PDF]


Alireza Abdollahi and S.M. Jafarian Amiri, Minimal coverings of completely reducible groups, Publicationes Mathematicae Debrecen, 72/1-2 (2008), 167-172 . 

 Alireza Abdollahi, Groups with maximal irredundant covers and minimal blocking sets, to appear in  Ars Combinatoria.