The element Knock is quite common in place names in Ireland, e.g. Ballyknock, Castleknock, Gortknock, Kilknock and Knockaderry [source]. There’s also quite a few places called simply Knock, the best known of which is the Knock in County Mayo in the west of Ireland , which is known as An Cnoc (the hill) or Cnoc Mhuire (Hill of (the Virgin) Mary) in Irish.

The Irish word cnoc (hill), from which Knock comes, is pronounced [kn̪ˠɔk] in Munster, [knˠɔk] in Aran, and [kɾˠɔk] in Galway, Mayo and Ulster. It comes from the Old Irish cnocc (hill, lump, swelling), from the Proto-Celtic *knokko(s) (hill), which is also the root of:

– Scottish Gaelic cnoc [krɔ̃xg] = hill, hillock, knoll
– Manx cronk [krɒnk] = mount, tor, hill
– Welsh cnwc [knʊk] = hillock, bump, lump, butte
– Cornish knegh [knɛx] / knogh [knɔx] = hillock

A similar, though unrelated, English word is knoll [nəʊl], a hillock or mound, which comes from the Old English cnoll (hill-top, cop, summit, hillock), which is cognate with the Dutch knol (clod, ball, turnip); the German Knolle (bulb, tuber); and the Swedish knöl (lump; bump; hump) [source].