Abortion has been illegal in Ireland from the middle of the 19th century until today, but this has obviously not stopped the practice from continuing. The methods in use to induce miscarriage historically have been varied, from different natural herbal preparations as /u/Kwyjibo68 has mentioned as well as the use of surgical means.

When looking at Ireland, specifically in the time period that the play An Triail was set in one of the best resources we have is the case of Mamie Cadden.

She was a midwife, born in America to Irish parents, who returned to Dublin and was well known for performing illegal abortions throughout the 1920s to 1950s. She was arrested and convicted multiple times, and in 1956 was sentenced to death for the murder of a women who had used her services and died as a result. The sentence was never carried out, but she died in prison.

She and other abortion providers in Ireland at the time were known to have given women ergot as one of the methods of inducing miscarriage (http://en.wikipedia.org/wiki/Ergot). It was in common use for centuries in Europe, but we have documented cases of its use in Ireland at the time.

Other methods that were employed were unlikely to have been the 'medicine' mentioned in the play, since they involved some surgical or clinical care (injections for example).

Source: The Lost Decade: Ireland in the 1950s by Dermot Keogh, Finbarr O'Shea, Carmel Quinlan (p. 158-162)

It was likely a natural abortifacient, usually an herbal mixture that induces a miscarriage. Substances used would include things like pennyroyal, blue cohosh, quinine, saffron, as well as many other herbs.

I'd chance posting this to r/ireland. Tonnes of people studied the play in school and might have discussed this before.