There are two entries that I can find in Fr Doyle’s diaries relating to March 21, both of which refer to his desire to sacrifice himself and perform mortification. The first one is dated 21 March 1913, which was Good Friday that year:

A. M. D. G. SOLEMN VOW. After much thought and prayer, feeling myself urged strongly by grace and the ceaseless pleading of Jesus, I have resolved to lead the life of absolute crucifixion which I know He wants and which alone will please Him.

I now promise and bind myself by vow (under mortal sin) ‘ to give Him everything until next Christmas Day, with the power of dispensing myself in case of necessity on any day.

Dear Jesus, I vow, with the help of Your grace, to give You all You ask for the future.

Good Friday, March 21st, 1913.

Three o’clock.

The second entry is for 4 years later, 21 March 1917, just 5 months before his death:

I know I can never be happy unless I am heroically generous with Him. This I have proved time after time. A sacrifice which costs much always brings great grace, joy and interior peace.

These words from 1917 probably capture Fr Doyle’s spirituality better than any others he ever wrote.