Ireland is still Celtic. Highland Scotland is as well.

Wales, Cornwall, and the Isle of Man are all Celtic.

Of those Ireland is still a Celtic country, while the Isle of Man is semi-autonomous. But you're specifically asking about Kingdoms.

The last Celtic king was Alexander III of Scotland, who died in 1286.

The last of the Brittonic kingdoms was Kingdom on Strathclyde, which fell in the 1090's. It was divided between England and Scotland.

Wales is a bit more confusing. Technically conquered by the Normans in 1094, they didn't respond well to the Norman conquest, and all sorts of uprisings erupted. It wasn't annexed to England until the Laws in Wales Act (aka Acts of Union) of 1535 and 1542. Before this Wales was more of a Principality, or a bunch of Principalities. Basically an unruly bunch that technically belong to the King of England, although they did not form a part of England itself, and the King exerted almost not control there, and various areas would declare themselves independent and form kingdoms, only to be put down again. The last Principality in Wales of any note was the Principality of Gwynedd, which fell to Edward the First by 1283. It's final ruler, Dafydd ap Gruffudd had declared himself the "Prince of Wales". Edward I kept that title and gave it to his son Edward II, and it's still a title held by the British monarchy.

Ireland has a story similar to Wales. Largely conquered by the Normans by 1172, parts remained independent and kingdoms would fight the Normans and amongst themselves. The last High King of Ireland was arguably Brian O'Neil, who died in 1260, but various other minor kingdoms survived until 1603. The last one to fall to the English was Kingdom of West Breifne, which was conquered in 1603. By 1605 the King of England was proclaimed as the King of Ireland as well.