The basics
What a solemniser actually does
In Ireland, a solemniser is the person legally allowed to marry you. Officiant and celebrant are the words most American couples search.
Solemniser is the Irish legal word. Wedding officiant and wedding celebrant are the words couples from the US usually type into Google. On this site they mean the same role: the person who conducts a legally valid marriage ceremony.
They must be on the Register of Solemnisers on the day they marry you. If they are not, the ceremony is not a legal marriage in Ireland.
They do not replace the HSE notice. They do not automatically include a venue, photographer or planner. Some will travel. Some will only work in their parish or office. The register does not always say which.