The Click Green article doesn't bother to link to REF, but the source is here, with this interesting introduction:

Low Wind Power Output 2010

In today's Times (02.02.11) it is reported that Scottish and Southern Energy (SSE) has published data confirming that its wind turbine fleet has reported a 20% reduction in energy generation in the last year. SSE is said to have released this data in response to requests from concerned shareholders.¹

This is consistent with data examined by Renewable Energy Foundation at the request of the Sunday Telegraph, which resulted in a report by Andrew Gilligan to the effect that the UK wind fleet load factor in the year October 2009 to September 2010 was very low in comparison to previous years.²

"Concerned shareholders" and Andrew Gilligan of the Telegraph only have to ask...

REF says it derives its 23.5% number from Eirgrid (no comment on the extremely narrow scale used to provide a dramatic graph - well, that's a comment...):

A footnote refers to:  Eirgrid (Jonathan O'Sullivan), "Facilitating the Transition to a More Competitive, Sustainable, and Low Carbon Electricity Future". Presentation to the Irish Renewable Energy Summit, 20th January 2011, Dundalk.

That presentation isn't online. The Summit has a site where Jon O'Sullivan is mentioned as doing a presentation with that title; but in fact the Speakers' Panel doesn't contain Jon O'Sullivan, and in the PDF brochure the presentation is down to Dermot Byrne, also of Eirgrid. But let's suppose the presentation was done for the conference (where it was followed by REF boss John Constable on wind variability).

Eirgrid offers system data on its site, with a page for Wind Generation. You can download generation stats (at quarter-hour intervals) for the dates you choose. But you don't get (or I didn't find) electricity delivered in MW/h.

As to capacity, the Wikipedia article is weird, since its own reference states:

Irish Wind Energy Association - Wind Energy in Ireland

The current* grid connected and operational installed wind capacity on the island of Ireland is 1746.7 Megawatts (MW) which will on average generate 4,743,339 Megawatt hours (MWh) in a year given a 31% load**or capacity factor.

... *Figures correct on 19/07/10.

According to a report (pdf) by the Sustainable Energy Authority of Ireland and Eirgrid, total island capacity (NI included) in 2011 is 2,163 MW.

That report is well worth reading in itself (it's quoted in the Wikipedia article re wind lowering costs in such a way as to cover FIT costs). But it doesn't give 2010 capacity. So the 23.5% still needs more digging to be validated.