Histogram with multiple plots of same data

First, note that your example gives an error, I think due to the combination of symbolic x coords={<something>} and xtick={<something else>}. The xtick key is for defining the x-values where you want to have a tick, but your list of ticks is different from the coordinates you have defined with symbolic x coord. Perhaps you were after xticklabels={...}, the purpose of that is to define the actual text printed as labels.

But back to the question. This is a little bit hacky, but it does give you the output asked for. I removed the symbolic x coords bit, I think it makes it easier if you use numeric coordinates (which you had in your addplot anyway). I use xtick to add ticks between the bars, at appropriate places, and then I use extra x ticks to add labels between the ticks.

documentclass[border=5mm]{standalone}
usepackage{pgfplots}
% you already have some compat setting in your document, so please keep that, and remove the following line
pgfplotsset{compat=1.3}
begin{document}
begin{tikzpicture}
begin{axis}[
    ybar=5pt,
    bar width=17pt,
    ylabel={Repair time (months)},
    symbolic y coords={$<$ 1 month, $>$ 1 month, 3 months, $>$ 3 months},
    ytick=data,
    % set x-limits explicitly
    xmin=0.5,xmax=6.51,
    % add ticks at appropriate x-values
    xtick={0.5,1.5,5.5,6.5},
    % don't print any labels at the default ticks
    xticklabels={},
    ymajorgrids=true,
    % add some extra ticks where you want the labels
    extra x ticks={1,3.5,6},
    % set the labels
    extra x tick labels={IL2,IL3,IL4},
    % hide the ticks themselves
    extra x tick style={tickwidth=0}
    ]
addplot coordinates {(6,$<$ 1 month) (5,$>$ 1 month) (4,$>$ 1 month) (3,3 months) (2,3 months) (1,$>$ 3 months)};
end{axis}
end{tikzpicture} 
end{document}