Hi,
I have IRanges 1.13.8 installed:
   > sessionInfo()
   R Under development (unstable) (2011-11-21 r57721)
   Platform: x86_64-unknown-linux-gnu (64-bit)
   locale:
    [1] LC_CTYPE=en_CA.UTF-8       LC_NUMERIC=C
    [3] LC_TIME=en_CA.UTF-8        LC_COLLATE=en_CA.UTF-8
    [5] LC_MONETARY=en_CA.UTF-8    LC_MESSAGES=en_CA.UTF-8
    [7] LC_PAPER=C                 LC_NAME=C
    [9] LC_ADDRESS=C               LC_TELEPHONE=C
   [11] LC_MEASUREMENT=en_CA.UTF-8 LC_IDENTIFICATION=C
   attached base packages:
   [1] stats     graphics  grDevices utils     datasets  methods   base
   other attached packages:
   [1] IRanges_1.13.8     BiocGenerics_0.1.3
And, surprisingly, I have no problem installing a package that
requires IRanges >= 1.13.9 (thru the Imports field):
   hpages at rhino1:~/HTSeqGenie/svn> grep IRanges RNASeqGenie/DESCRIPTION
   Imports: BiocGenerics (>= 0.1.3), IRanges (>= 1.13.9),
   hpages at rhino1:~/HTSeqGenie/svn> R-2.15 CMD INSTALL RNASeqGenie
   * installing to library ?/home/hpages/R-2.15/library?
   * installing *source* package ?RNASeqGenie? ...
   ** R
   ** inst
   ** preparing package for lazy loading
   ** help
   *** installing help indices
   ** building package indices ...
   *** tangling vignette sources ...
      ?RNASeqGenie.Rnw?
   ** testing if installed package can be loaded
   * DONE (RNASeqGenie)
Thanks!
H.
-- 
Herv? Pag?s
Program in Computational Biology
Division of Public Health Sciences
Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N, M1-B514
P.O. Box 19024
Seattle, WA 98109-1024
E-mail: hpages at fhcrc.org
Phone:  (206) 667-5791
Fax:    (206) 667-1319