I wouldn't read too much into the filing of the M1 form, as any director or Secretary can sign the form and file it. The real question is "Why" was this done? It may simply have been in error, or it may have been registered against the wrong company number (this does happen!), or there may be something more sinister going on. Do you have a share certificate for the shares you hold? If not, what confirmation do you have that you ever were a shareholder? Were you listed as a shareholder on the last annual return (or accounts, if any were filed)? Are there other shareholders? Is there a shareholders' agreement? Did you ever sign a Stock Transfer Form, and if so, who retained it? Have you downloaded a copy of the M1 from the CRO site, and if so, are the company details correct, and who signed it? Have you contacted the company to discuss this issue?
As I said, the filing of the M1 is only a detail, you need to get behind what is going on at the company. There is a strong remedy available to minority shareholders whose rights are being oppressed (see s.205 Companies Act, 1963), but you would need to be clear on all of the above points before taking the issue further with a competent company law solicitor.