As the book itself explains it is ...
"The Church Catechism in Irish, with the English placed over against it in the same Karakter"
Here is the description given by the Linen Hall library ...
"The Church Catechism was commissioned by the Bishop of Down and Connor, Francis Hutchinson, in an attempt to convert the Irish-speaking catholic population of Rathlin Island, Co. Antrim, to Protestantism. Written phonetically, the Catechism is a valuable record of spoken Ulster Irish in the early part of the eighteenth century."
It is inaccurate to state that the Catechism is written 'phonetically'. It is in fact written in Roman type in a spelling system based to a large extent on English rather than traditional Gaelic spelling, the effect is strikingly similar to Manx.
The book is of great religous, historical and linguistic significance not only for Rathlin but for all of Ulster.
This is the academic reference.
Ó Dochartaigh (Cathair): The Rathlin Catechism.
In ZCP 35 (1976), pp. 175–233.
Ed. of Irish sections of The Church Catechism in Irish, printed in belfast by James Blow in 1722: includes Catechism, Tegasg Kreesdee; prayers for the sick, Oornai ar son Yhaoniv Tynn; dialogues, Kolavara; and vocabulary. Orthography based on literary Irish with considerable modifications in direction of English. Published as tool for teaching Rathlin parishioners the English language. Detailed linguistic, including dialect, analysis.