Deadly

adj.

Heard in Fermanagh, Kilkenny, Wexford, Dublin, Cork, Galway, Waterford, Tyrone and Tipperary

Brilliant, fantastic, or exceptionally good.

In use

Dat fiddle player down in de hall was deadly, hi!

Posh English

That violinist performing in the hall was simply first rate, my good fellow.

Same phrase, other counties