It seems as if Ryan O'Reily was feared and respected in Oz, partially due to him being Irish in general and being in a gang on the outside.
Aside from the O'Reily brothers, there aren't any real significant Irish inmates. At the beginning of season 2, they are listed as a group in em city and had O'Reily, Timmy Kirk, and a few other background characters, but throughout the series Ryan is mostly on his own and embodies the Irish in oz.
I could be wrong but I think the Irish were already a healthy portion, just poorly organized and dispersed. They weren't as uniform as the Homeboys or Aryans. there's alot of Irish on the east coast so I wouldn't be surprised they could equal out against the other gangs if they had actually put in effort to recruit their ranks.
also, even if the Irish were a large-set gang I doubt O'Reily would become part of the clique, regardless. he's too independent-minded. he's more of the Rugged Individualist / survivalist type.
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S = K Log W