newcommand{reqV}[7]
{
  multirow{5}{*}{#1} & subReq{#1.1}{#3}   & multirow{5}{*}{#2} \
                        cline{2-3}
                      & subReq{#1.2}{#4}   &                     \
                        cline{2-3}
                      & subReq{#1.3}{#5}   &                     \
                        cline{2-3}
                      & subReq{#1.4}{#6}   &                     \
                        cline{2-3}
                      & subReq{#1.5}{#7}   &                     \
}

documentclass{article}

usepackage{tabularx}
renewcommand{tabularxcolumn}[1]{ m{#1} }
newcolumntype{M}[1]{ >{centeringarraybackslash} m{#1} }

newcounter{req}
newcommand{req}[3]{%
  setcounter{req}{1}%
  parnoindent
  begin{tabularx}{linewidth}{ | M{5em} X M{7em} | }
    hline
    #1 &
    {letoldbackslash\
     renewcommand{tabularxcolumn}[1]{ p{##1} }%
     begin{tabularx}{linewidth}{ | @{hspace{tabcolsep}#1.thereqhspace{tabcolsep}} | X | }
       globaldef\{stepcounter{req}oldbackslash hline}
       #2
     end{tabularx}%
     globallet\oldbackslash} &
    #3 \
    hline
  end{tabularx}
  par
}

begin{document}

req{IR5}{A \ B \ C \ D}{Mandatory}

req{IR6}{%
  Lorem ipsum dolor sit amet, consectetur adipiscing elit. \
  Pellentesque eget nisi non tortor tincidunt porttitor. \
  Aliquam eget sem arcu. \
  Suspendisse potenti. \
  Nullam pellentesque sapien vel leo consequat facilisis. \
  Aliquam pretium convallis arcu, ut volutpat massa pellentesque quis. \
  Praesent dignissim facilisis risus ac dignissim. \
  Interdum et malesuada fames ac ante ipsum primis in faucibus.}{Optional}

end{document}