$\mathrm{Ce}, \mathrm{Pr}$ and $\mathrm{Nd}$ are lanthanoids and have incomplete $4 \mathrm{f}$ shell while $\mathrm{Th}, \mathrm{Pa}$ and $\mathrm{U}$ are actinoids and have incomplete 5f shell.
The 5 f-electrons are more effectively shielded from the nuclear charge than $4 \mathrm{f}$ electrons of the corresponding lanthanoids.
Therefore, outer electrons are less firmly held and they are available for bonding in the actinoids. Due to which I.E. of $\mathrm{Ce}, \mathrm{Pr}$ and $\mathrm{Nd}$ are higher than Th, Pa and $\mathrm{U}$.