Heian-kyo was pronounced as 'Heian-kyo' in On-yomi (Chinese readings) by later generations, but it was originally pronounced as 'Taira no Miyako' in Kun-yomi (Japanese readings).
平安京は後世においては音読みの「へいあんきょう」と読むが、当初は「たいらのみやこ」と訓読みした。