I have found birth certificates for a McTague family in Balbriggan, Dublin on RootsIreland.ie (a paid subscription site). The parents, Michael and Jane (nee Grimes) had several children in the 1840s, one of them being a John McTague. According to his birth certificate, he was born in 1847 and was Roman Catholic.
I cannot be sure that these are the McTagues you are looking for, as the fathers name differs from the one you have. However, it may be a lead for you? Knowing the parish/area of the family and their religion can be very helpful in these searches, as many birth certificates can be found through the parish.
Also, to note as you mentioned you are new on the site, there is an option on this site to "add an ancestor" so you can add your great-grandad if you would like. This might help others who may be connected find you.
Please let me know if I can be of any more assistance, or if there is any more information that may help in the search.
I wish you luck!