Employer is either confused or not telling the full story.
The specific change that affects your girlfriend is the 2003 EU Pensions Directive, which was implemented in UK legislation in 2005 and 2006. As a result, it is a bit more complicated for a UK pension scheme to have Irish members than it used to be. It is still allowed, but some employers have decided that its not worth it.
There is no clear or easy answer to your second question, about her entitlements, and she probably needs legal advice. Her contract of employment may or may not mention pensions, and if it does, it may or may not be specific about the type or amount of pension provided. The employer may therefore have the right to change the pension, but on the other hand, she may be able to argue that they are worsening her overall terms of employment.
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